Actually, chair cushion re-upholstery is quite easy.
You remove the seat from the chair, remove all of the staples, and then remove the fabric.
Use the fabric piece as a pattern to cut the new fabric to size. Add a little extra in any place where it didn't seem to be quite enough when they originally did the upholstery. It can always be trimmed off after installation.
Then position the new piece of fabric face down on the bench and lay the plywood and padding upside down on top of it. Now, position the plywood so it's centered on the fabric. Then pull the fabric up over the padding and plywood and install one staple in the center of that side.
Now rotate the plywood and fabric 180 deg and pull the fabric tight to the tension desired and place another staple in the center of that edge of the seat opposite where you placed the first staple. Always work from the center out, keeping the tension even as you work opposite sides. Check to be certain that you aren't creating wrinkles between the opposite side staples as you work. Wrinkles in the un-stapled corners can be removed as you work out to them adding staples as you go.
Now rotate the seat 90 deg, pull the fabric to the desired tension and place a staple in the center of that edge. Then rotate the seat 180 deg, pull the fabric to the desired tension and place staple number 4 in that edge.
Flip the seat over and check that the fabric is centered and at the correct tension between the 4 stapled points. If not, you need to go back and correct this.
Once happy with the positioning and tightness, you can flip the seat over and continue adding staples, working out from the center staple in each side and pulling the fabric to the same tension. It is very important to keep rotating the seat 180 deg after each staple is installed and keep working out toward the corners in an even staple installation pattern.
As you approach the corners, figure out how to fold the fabric around the corners to match the way that the original fabric had been folded. Then fold it and staple it in this same pattern, again working on opposite corners like you did previously, to keep the fabric centered and even tension.
When the seat has been completed, re-attach it to the chair frame.
Once you have completed the re-upholstering of the first chair, you will be able to do the rest the same way, but you will quickly become much faster at doing it. The real trick to doing chair upholstery is to keep working on opposite sides to keep the fabric centered and of equal tension. The corners can sometimes be tricky to fold as there are several methods possible, but duplicating the way that it was originally folded is usually the best for that piece of furniture.
If the padding has to be replaced, buy the same padding material as the original and use the original as an approximate pattern to cut it to size. In most cases, for a chair seat, the padding shape will match the dimensions of the plywood.
Since you will be installing a lot of staples, a pneumatic stapler is strongly suggested. Also, be very careful to use the correct length staples. If too short, they will not stay in. If too long and they go through the plywood seat, They will be very uncomfortable to sit on.
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