Remove the seat from the chair using a Phillips head screw driver. Flip the chair over for easy access to the screws on the underside of the seat.
Use a staple remover to take out the staples securing the fabric onto the dining chair seat. Remove the fabric and set it aside. Remove and dispose of the old batting and foam inside the seat.
Measure a new piece of foam or batting. Choose batting or foam of 1-inch thickness. If using a combination of the two materials, use 1/2-inch batting on top of 1/2-inch foam. Cut the pieces so that they are 1/4-inch longer than the chair seat on all sides.
Place the foam on the seat and cover with the original fabric. Flip the seat so the foam is on the bottom and pull the fabric tightly on all sides.
Staple the fabric back onto the seat while stretching the fabric across the foam to keep it tight. For corners, fold the fabric before stapling, causing a tight crease on the underside of the chair seat.
Reattach the seat to the dining chair by affixing the screws with the Phillips head screw driver. Turn the screws as tightly as possible to ensure the seat stays secure.