Cut your rush into 30-foot lengths for easier handling. Place a bucket of water next to your work area. Dip your roll of rush into the water for 20 seconds. Tack the end of the rush to the left inside rail of the chair 1-inch from the left front leg.
Bring the rush up and over the front rail, under and up to the right of the rush. Go over the rush to the left and over the left side rail, under and up in the center front of the seat. Go to the right, taking the rush up and over the right side rail, under and up crossing over the rush and front rail to the underside. Tack the end of the rush 1-inch from the right front leg.
Start your second weave by tacking the next piece of rush 1 1/2 inches from the left front leg along the inside of the left side rail. Follow the same weave pattern keeping the rush tight against the first weave of rush. Tack the end off to the right underside rail 1 1/2 inches from the right front leg. Continue your front weave until your front weave width is even with the inside of your rear leg. Most chairs are wider at the front. Your front weave compensates for this extra width.
Start your full weave by tacking a new length of rush close to the left rear leg on the inside of the left side rail. Weave the front weave but do not tack off to the right side. Bring the rush up and over the back rail close to the right back leg. Take the rush up and over the rush to the right and over the right side rail, under and up to the left. Bring the rush over the left side rail, under up and over the rush to the back and over the back rail. Repeat the weave for five passes.
Measure the width of the seat between the back legs of the chair. Subtract 2 inches. Cut a cardboard triangle to this size. Cut the triangle in half, cutting the top of the triangle off. Discard the top of the triangle. Mark the remaining piece as the back. Repeat this for each side and the front. Label each. Cut two more identical pieces of each side so that you have three filler pieces in each shape. Insert the three filler pieces between the rush layers with the points toward the chair legs. These do not need to be precise, the cardboard simply helps support the seat.
Block the rush by placing a wood block on the rail next to the rush and hammering on the wood against the rush to tighten the rush toward each chair leg. This should be done every four or five weavings. Tie new strands to old strands on the back, left, underside of the weave. Keep twisting the rush in the twist direction to keep it strong as you weave. Weave and block until only a 1-inch space in one direction remains open.
Weave a figure eight to finish your chair seat. Take the rush up and over the front rail. Bring the rush up in the middle and over the back rail and back down to the underside. Repeat. Continue the figure eight until the chair seat is complete. Tack off the ends to the closest inside rail.