Prepare the rushes by soaking them for long enough that they will twist without cracking. Store them together in a cloth bag while you work to prevent them from drying.
Tack the first strand of rush to the interior of the left seat rail, about two inches back from the front seat rail. Draw the rush toward you and over the front seat rail, immediately next to where it meets the chair post. Wrap the rush underneath the front rail in a loop, then over the left seat rail, again next to the chair post. Loop the rush underneath the left seat rail, and then draw it across the seat, next to the front seat rail.
Wrap the rush over the right seat rail, next to the post. Loop it under the rail, then pull it over the front seat rail, next to the post, and loop it underneath the rail. Tack the rush to the interior of the right seat rail, about two inches back from the front seat rail, and cut off the excess.
Repeat the process to form a square around the seat rails, tacking the rush to the interior of the seat rails and continuing from left to right around the chair. Always loop the rush in the same fashion, over and then under the rail. Press each successive row tight against the last.
If desired, add cardboard filler between the layers of rush when you have woven about three inches from each corner. Cut the cardboard pieces into trapezoid shapes, with the bottom, longest side of the trapezoids an inch or two shorter than the chair rails. Tuck the corners of the trapezoids into the rushes you have already woven, with the long side parallel to the seat rails. Continue weaving around the exposed cardboard.
Continue weaving until you reach the center of the chair. Complete the seat by tacking the end of the rush to the interior of a seat rail.
Finish the seat with a light protective varnish.