You must remove the seat cushion from the chair frame. The covering is most likely stapled to the bottom. Remove the staples and cover. Trace a pattern from this onto the new material chosen. Cut out and restaple the material to the seat form fitting it snuggly as you go.
Upholstering dining room chair sets can be simple. Turn the chair over and locate the screws. Use a screwdriver to remove the screws and detach the seat from the chair frame. Place the chair seat on your choice of fabric. Fold the fabric over and secure with heavy duty staples. Reattach the seat to to the chair using the screws. Repeat.
You don't need instructions. Take the material off your seats and cut the material of your choice the same size of the material you took off. Get a staple gun. Staple each corner and then go around the seat and staple. If you need instructions go to the library and get an upholstery book.