I absolutely hated the material on the seats of my dining room chairs. I went to a whole bunch of design stores and upholstery shops, only to find out that it would be a pretty significant chunk of change just to get them all recovered. After doing a little research, I figured that, with a little time, effort, and some help from my friends at Techshop, I could recover my set for under $50. I MADE IT AT TECHSHOP!

http://www.techshop.ws

Step 1: Remove the Seats From the Chairs

This step is simple. Turn your chairs upside down and figure out how to get the seat cushions off of the chair. Mine were just screwed down.

Step 2: Remove the Old Fabric

Again, a simple step. The material covering the chair should just be held down by staples. Remove them all and keep all the pieces for later use.

Step 3: The New Material

Next, find a good material that you want to cover your seat cushions with. Make sure to find something that is durable. After all, it's a high-traffic piece of furniture that needs a sturdy cover that won't wear out. I recommend getting about a half a yard per seat cushion. Then, use the old material pieces that you just took the staples out of as a stencil to mark and cut out the new seat cushion covers from the new material.

Step 4: Attaching the New Material

The next step is to take the new cover and wrap it around the old cushion and backing. Line up the cushion, the backing, and the material to where it all fits together like it did in the original cover. As a side note, if the material has a pattern on it, make sure that the pattern is lined up with the cushion before you start to attach the material to the cover. The first time I tried this, I ended up making the pattern at an angle, and it looked pretty terrible. A trick to make it easier is to line up the old cover on the new fabric so that way when you cut it out, it will be lined up already instead of having to adjust the material to sit right on the cushion.
After this, what I did was put a staple in the center of every side, putting slight tension on the fabric to hold it taught. Use a simple hand staple gun for this with staples that are about 3/8 inches long or so. Then, finish the lines of staples on the two sides of the seat. The front is the first part of the seat people see, so it will be the last bit to do so that it covers everything else. Then attach the edge of the fabric in the back. Finally, attach the fabric to the front. It does not matter how far apart the staples are; separating them about two inches or so will do. Also, do not pull the fabric too tight. This causes indentations on the side where the fabric pushes down on the cushion.
Remember, the attachments don't have to be pretty. No one sees the bottom of the cushion!

Step 5: Attaching the Corners

This one's the tough part. I had to work the material on the corners into several arrangements until I was satisfied. It took patience and effort, and all the corners looked different. Just as a pointer, there's no right or wrong way to do it, but make sure that each side mirrors the other. Pull the fabric tight, and put a whole bunch of staples down to hold the corners tight. Work the wrinkles on the sides out before you put the staples down.

Step 6: Adding Piping Around the Edge

I did not do this, but some chairs come with piping around the edges. The way to do this is by taking a thick rope (the thickness is up to you) and wrapping material around it. Sew a line immediately adjacent to the rope to hold it in place. Leave about a one or two inch lip to staple the piping around the edge of the seat.

Step 7: Attaching the Cover

The netting-like black piece that was in the first picture may not be on your old seat cushion. This is just a cover to hide the edges of the fabric. If your chair came with one, use the old one to cover the edges of your new fabric. If not, it's not necessary. You can also buy a new fabric piece to cover the edges if you like.

Step 8: The Final Product

This is what I ended up with. The material completely covered the cushion, there were enough staples around the edge to hold the material in place, and the pattern lined up well with the edges of the cushion.