Most windows need a little help from window coverings to manage light levels or provide privacy. But sometimes it’s just about making them look their best.

Window coverings “absolutely add a finishing layer to a space,” said Augusta Hoffman, an interior designer in New York. “A room doesn’t feel finished until you add them.”

Usually, the easiest way to add that layer is by installing shades or blinds. But there are so many styles, materials and installation choices available. How do you decide what’s best for your room?

Review Your Options

Roman shades are a designer favorite because they are made of fabric, so they offer a softening look similar to that of curtains, without dragging on the floor or getting caught up in the furniture.

“I always find it problematic when curtains are not able to function,” said Gideon Mendelson, the designer who founded Mendelson Group, in New York. “That drives me bonkers.”

That’s why he often uses Roman shades in his projects. “You might have a radiator, a heating unit or other enclosure under a window that prevents curtains from closing,” Mr. Mendelson said. “In that case, a Roman shade works better.”

For a more minimalist look, some people prefer a simple roller shade or a cellular shade, said Adam Skalman, the vice president of sales at The Shade Store.

“A roller shade has a super streamlined, minimal, modern aesthetic,” he said, while a cellular shade has a honeycomb structure that expands when the shade is lowered, so it can be tucked discreetly out of the way when not in use.

“That honeycomb fabric provides insulation as well,” Mr. Skalman said, so it might be a good choice for drafty windows.

A more casual, natural-looking option is a woven wood or matchstick shade, which is made with thin strips of wood, bamboo or natural grasses. “Woven wood is my favorite material,” said Ms. Hoffman, who has installed shades from Hartmann & Forbes in many of her projects. “It instantly brings a kind of organic texture.”

Blinds, on the other hand, are made from individual slats of wood, metal or plastic, rather than a single sheet of material. “They have kind of a retro reference,” said Everick Brown, who runs Everick Brown Design with his wife, Lisa Walker Brown.

The couple installed wood blinds in the kitchen of their home in Yorktown Heights, N.Y., both for the handsome look and the functionality. “I love that you can change the tilt, which creates multiple opportunities for filtering light as the sun rises and sets,” Ms. Brown said.

Choose Color and Pattern

Once you’ve chosen a type of shade or blind, find a material with the color, pattern or texture you want. If there are eye-catching decorative elements in the room — or if you simply prefer a sense of calm — the best choice could be a solid color that complements the walls.

Christina Nielsen, an interior designer in New York, chose off-white Roman shades with a subtle ticking stripe from Wovn Home for a living room she designed with textured plaster walls. “I wanted something that blended rather than detracted from the walls,” she said, “because when you walk into the room, I want you to notice the plaster.”

In a hallway with grasscloth on the walls, she chose a Roman shade with a buff-colored, textured fabric for the same reason — when the shade is down, it almost looks like a continuation of the wall finish.

For a room that needs a little decorative oomph, a material with a contrasting color or expressive pattern may be a better choice. Mr. Mendelson sometimes uses Roman shades with bright, graphic patterns in children’s rooms or spaces that may otherwise look a little too neutral.

When designing a kitchen for a country house in England with white walls and sage-green cabinets and shelves, Ms. Nielsen added Roman shades made from a pink floral-patterned fabric. “I wanted to play up that English aesthetic with more of a lived-in feel,” she said. She used the same fabric to line the backs of the shades, giving them a decorative punch when viewed from the yard outside.

How Much Opacity Do You Need?

Different materials block different levels of light. When you’re choosing from a range of options — from thin, light-softening sheers to blackout shades — consider not only how much sunlight the room gets, but also your preferences.

For bedrooms, some people prefer blackout shades, while others like to see a little light as the sun rises. In living rooms with ample sunlight, you may need a material that filters a lot of light, if not a blackout shade; in rooms that don’t get a lot of direct sunlight, a more transparent material is a better choice.

Ms. Hoffman sometimes uses shades made with a light, gauzy cotton or linen in rooms where light control isn’t a concern. “Those are completely sheer, super-light textural fabrics,” she said. “It’s really just a way to diffuse light and finish out the room.”

If you plan to buy Roman shades, they can be ordered with a light-filtering or blackout lining to adjust the amount of light they block. Roller and cellular shades are also available in various levels of opacity, and woven wood shades can do the same thing by employing a tighter or looser weave.

It’s essential to obtain swatches before ordering new window coverings. “Take those swatches and tape them up on the window to evaluate them at different times during the day,” Mr. Skalman said. And if you’re considering using lined Roman shades, be sure to put a sample of the lining behind the fabric swatch.

Just remember that the more transparent the material, the less privacy the shades will provide at night, so it’s worth looking at them from the street at night, with the lights on.

Blinds don’t require so much investigation. When they’re closed, they offer an effect similar to that of a blackout shade; when open, they provide almost total transparency. In between, they can be used to modulate sunlight.

Consider Using Layers

What if you like the look of one type of shade, but need the functionality of another? In that case, some designers use two layers of window coverings.

Ms. Hoffman often installs a woven wood blind facing the room, for its decorative appeal, then tucks a roller shade behind it. The roller shade can be dropped down when a darker room, or more privacy, is desired. Mr. Brown has installed wood blinds behind translucent Roman shades for a similar effect.

The reverse approach to layering also works, Ms. Hoffman said: If you have a heavy, lined Roman shade in a room, you can mount a lighter roller shade behind it for when you just want to diffuse the light.

Choose the Controls

The more often you open and close your shades or blinds, the more important the controls are. The simplest control is a string-pulling mechanism with a cleat. But these sometimes require a little muscle, and it can take more time to keep them from parking askew. Strings can also become tangled and may pose a safety hazard.

Some window coverings are available with a manual cordless mechanism, which allows the shade or blind to be raised and lowered with a hand push, making operation easier while offering a clean, uncluttered look. Another option is a continuous loop mechanism with a beaded chain that cranks the window covering up or down when pulled continuously in one direction.

Motorized shades and blinds are increasingly popular, though more expensive than manual options. Many manufacturers now offer battery-powered shades and blinds, so the window coverings can be installed without having to run new wiring, and the movement can be automated with a remote control or smart-home app. The upside is that you don’t need to pull strings or chains; the downside is that the batteries need to be periodically replaced or recharged.

Plan the Installation

There are two strategies for mounting shades and blinds: Install them inside the window frame or place them on the outside.

Mounting them inside the frame offers an integrated, custom look and leaves the surrounding window casing exposed. “It feels more considered and like you took the proper dimensions,” Ms. Hoffman said.

That’s a point worth emphasizing: Measurements are critical when you’re ordering inside-mounted shades. Vendors like The Shade Store, Smith & Noble and Blinds.com recommend measuring the actual size of your window opening in multiple locations up and down the frame, because windows are not always perfectly square or rectangular. From those measurements, most companies make automatic deductions for the size of the shade or blind, to give it a little breathing space inside the frame.

But sometimes a window frame isn’t deep enough to hold an inside-mounted shade or blind — or you may prefer the look of an outside-mounted window covering. Ms. Nielsen has used outside-mounted shades to hide window casings she didn’t like. When you’re mounting a window covering outside the frame, it’s common to size the shade or blind so that its edges align with the outside edges of the window casing.

It is also possible to make a small window appear taller, Mr. Mendelson said, by placing an outside-mounted shade at the ceiling or the bottom of your crown molding, so it covers the wall space above the top of the window. “That illusion is one of the most important things I learned about window treatments,” he said. “It’s a way to add verticality to a room.”

Of course, if you plan to use two layers of shades, you could use both installation strategies, with a decorative shade on the outside of the window frame and a second shade tucked inside the frame.

“It’s like a stage where you’re able to create different moods and moments,” Mr. Brown said. “The more you layer, the more you’re able to do that.”

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