Before you begin, gather the following materials and tools:

Materials:

- Wood (Plywood or MDF): This will form the structure of the armrest.

- Cushioning/Foam: For comfort, especially if the armrest is going to be used frequently.

- Upholstery fabric: To cover the armrest and give it a polished, finished look.

- Wood glue: For strong adhesion of wood pieces.

- Screws or nails: To assemble the armrest components.

- Staple gun: For attaching fabric to the wood frame.

- Batting or padding (optional): For added cushioning on the armrest.

- Wood stain or paint (optional): For finishing the wood to match the sofa’s color and texture.

Tools:

- Measuring tape: To ensure precise measurements.

- Saw: To cut the wood to the required dimensions.

- Drill: For creating holes for screws or nails.

- Screwdriver or drill driver: For assembling parts.

- Staple gun: To attach fabric to the wood.

- Sander or sandpaper: To smooth edges on the wood.

- Clamps (optional): To hold pieces together while the glue dries.

The first step is to determine the size and shape of the armrest. Armrests typically range from 6 to 10 inches in height, and they should match the overall scale and style of your sofa. Measure the length of the side of your sofa where the armrest will be placed and decide how wide you want it to be. You can make the armrest as simple or as intricate as you like, depending on your desired style.

- Height and Length: 

Ensure the armrest is proportional to your sofa. It should be high enough to offer comfortable support but not too bulky.

- Shape:

The armrest can be straight, rounded, or angular, depending on the look you're going for. If you're designing a modern, minimalist sofa, you may prefer a sleek, straight armrest. For a more traditional look, consider a rounded or cushioned armrest.

Once you have your measurements, it’s time to cut the wood for your armrest. You can use plywood, MDF, or hardwood, depending on your preference for durability and weight. Here's how to go about it:

- Base:

Cut a piece of wood that will serve as the main structure of the armrest. The dimensions should match the length of the side of your sofa where the armrest will be installed.

- Top and Sides:

For the top, cut a piece of wood that is the same width or slightly narrower than the base, depending on your design. The sides of the armrest should be cut to the height you want the armrest to be.

- Support Pieces:

 If your design includes any curved or angled features, you may need additional pieces of wood to provide support and stability. Use a jigsaw to cut curves or angled edges if needed.

Now that the pieces are cut, it’s time to assemble them. Follow these steps to build the armrest structure:

- Attach the sides:

Start by attaching the side pieces to the base using wood glue and screws. Make sure the edges are aligned and square. Use clamps to hold the pieces in place while the glue dries. If you're using screws, pre-drill holes to avoid splitting the wood.

- Install the top:

 Once the sides are secure, attach the top piece of the armrest to the frame. If you want a solid, sturdy armrest, make sure the top is attached firmly to the sides. Again, use wood glue and screws to secure it.

- Reinforce with additional supports (optional):

If you want extra durability, you can add internal supports or brackets to prevent any wobbling.

- Sand the edges:

 Once the structure is assembled, use a sander or sandpaper to smooth the edges and corners of the armrest. This will make the piece safer to use and give it a more polished appearance.

To make the armrest more comfortable and soft, you can add cushioning or foam padding.

- Cut the foam:

Use a foam cushion or foam sheets that are slightly smaller than the top of the armrest. Cut the foam to the appropriate size and shape. If you're using foam sheets, you can layer them for additional comfort.

- Attach the foam:

Use spray adhesive or foam glue to attach the foam to the top of the armrest. Be sure to press the foam down firmly so that it adheres well and doesn't shift over time.

- Add batting (optional):

If you want a softer, more plush feel, you can wrap the foam in batting before applying the fabric. Batting helps to smooth out any edges of the foam and gives a professional, smooth finish.

Upholstering your armrest will give it a sleek, finished look while also protecting the foam padding.

- Cut the fabric:

Choose a fabric that complements your sofa and is durable enough to withstand regular use. Measure the length and width needed to cover the top, sides, and any curved areas. Leave extra fabric around the edges for folding under the armrest.

- Staple the fabric:

Start by stapling the fabric to the underside of the armrest frame. Stretch the fabric tightly across the surface and pull it around the edges. Use a staple gun to attach it to the wood underneath. Work your way around the entire piece, making sure the fabric is smooth and free from wrinkles.

- Tuck and finish:

Once the fabric is stapled in place, tuck the edges under and staple them securely. For a neat finish, fold the corners as you would wrap a present. This will give the armrest a clean, professional look.

Finally, it’s time to attach your armrest to the sofa. Depending on your design and sofa construction, there are a few ways you can do this:

- Screw the armrest directly onto the sofa frame:

If your sofa has an exposed frame, you can use screws to attach the armrest securely to the side of the sofa. Pre-drill the holes to avoid damaging the wood.

- Use adhesive strips or brackets:

If you want to keep the armrest removable, you can use strong adhesive strips or brackets that will allow you to attach and detach the armrest as needed.

Once your armrest is installed, you can add any finishing touches to match the overall aesthetic of your sofa and room. You may choose to paint or stain the wood frame to match the existing sofa color or decor. You can also add decorative trim or buttons for a more stylish, upholstered look.