Purchase a piece of 1/2 inch foam at a fabric supply store or crafts retailer.
Place the foam in the seat of the rocking chair and trace a rough outline of the seat on the foam with a marker. This does not have to be perfect; make marker lines slightly smaller than the rocking chair's seat.
Cut the foam to size using scissors. Then place the cut foam on the rocking chair seat to ensure that it fits. You want the foam to cover the seat, with just an inch clearance on each side.
Roll out a fabric of your choosing on a table with the back side of the fabric facing up, then put the foam on the fabric.
Fold the fabric over the foam and fold the ends together.
Sew the fabric together with the foam inside using a needle and matching color thread. Cut excess fabric off using scissors. Turn the cushion over and place on the rocking chair seat.