Decide whether you want to make an L-shaped seating area around two sides of a table, a niche booth with a U-shaped seating area around three sides of a table or a two-sided booth with fixed benches on the two opposite long sides of the table. Draw up a test floor plan to scale to help you decide.
Construct the booth seating banquettes or benches. Determine the ideal seat height for your dining table. For most tables, this is 18 inches from the floor to the top of the seat of the chair. Use 2 inch by 2 inch dimensional lumber to construct the skeleton of a box. The box should be the proper length for the banquette as measured along the wall. The box should be about 26 inches wide, from the front of the bench to the back. The box should be 14 inches tall from the floor to the seat of the bench. These dimensions will accommodate the seat cushions.
Sheath or cover this frame with ¾ inch thick plywood or medium density fiberboard (MDF). Use plywood sheathing if you intend to upholster the bench completely. Use MDF if you want to paint the bench. If the back of the bench will never be seen, don't sheath the back.
Build a backrest for the bench. Build another box frame, equal to the length of the bench. The backrest frame should be no more than 6 to 8 inches in width. The backrest can be as tall as you like: mid-back height, shoulder height, or as tall as an average person is when seated. Sheath it in 3/4 inch thick plywood or MDF.
Mount the backrest on the back edge of the bench with wood screws. Drive the screws down through the framing of the backrest into the bench, or use angle irons.
Accommodate any turns for an L or U shaped bench. It is sometimes easier to build any "turns" as separate bench units and put them together at a later stage. This also produces a bench that is easier to manage in terms of size and weight. Conduct a 'dry run' of your bench units to see what modifications may be needed to fit the pieces together, snugly, into an L or a U.
Cover the front and the side panels of the bench units, and the side and top panels of the backrests in a suitable upholstery material, if desired. Pad these areas with 1 inch thick foam rubber, cut a little larger than the sizes needed for each panel. Staple the foam rubber to the bench before covering it with fabric. Roll the foam over edges and around corners to soften them.
Optional: if you want to simply paint the benches, cover all the corners and construction joints with molding trims. Prime the benches. Paint the benches with two coats of a durable, semi-gloss or gloss enamel paint.
Acquire large sheets or blocks of foam rubber from fabric stores or online. Choose the thickness of the foam according to your tastes: 4 to 6 inches thick is standard.
Cut the foam into sections (an electric bread knife works well) to match the size of your bench seats. Cut the foam slightly larger than the seat both along the front (under the knee) and on the sides. If you have an L or a U turn to span, make the L one piece of foam. Make two long straight pieces of foam to fit on either side of this L. This technique works with U-shaped benches as well.
Cover the foam seat blocks with a gauze or "facing" material available at fabric stores. Baste stitch the facing material so it encapsulates the foam blocks completely.
Upholster the seat blocks with fabric. Use a sewing machine to create six-sided "envelopes" for the cushions. Sew the fabric panels together inside out. Leave one "flap" of the envelope open, along a long edge. After sewing the envelopes, turn them right side out.
Insert the foam cushions into the envelopes and then close up the open end by whip-stitching the seam closed by hand. If you want to create washable covers, tailor the cushions with nylon zippers along the back seams so you can remove the foam cushion.
Place the seat cushions on the benches. Measure the distance from the top of the cushion to the top of the backrest. This is the height you need to make your backrest cushions. The seat cushions need to fit under the lower edge of the backrest cushions.
Construct backrest cushions as you did the seat cushions.
Attach the backrest cushions to the backrest with four vertical rows of heavy-duty Velcro strips.
Push the upholstered benches into the final position against the wall. Pull the table up to the benches and test the clearance needed to get in and out of the benches comfortably. Adjust the table location as needed.