Remove the seat and back material of a lawn chair, if they are still attached. An aluminum-framed folding chair with a woven plastic seat and back is ideal, though most varieties will do.
Lay a row of ties across the seat in parallel lines, alternating thick to thin ends to compensate for ties' irregular shape. Position a short end-table or tall stack of books temporarily underneath the chair for support, if the ties sag too much.
Insert another row at a right angle to the first, weaving the ties alternately over or under those already in position. Weave all the ties together in a simple over-under pattern, forming the seat. Once you've finished, you'll have a colorful mat that is strong enough to support your weight.
Tie the ends of the ties together underneath the chair to secure them. Since each tie is resting on top of the frame, tying it underneath will pull it tight and keep it from slipping. If the ties are long enough, you should be able to knot each necktie to its own end, though tying them to other ties will also work. Fasten them with square knots---cross the ends left over right and under, pull tight, cross right over left and under, and tighten again. There should be no loose ends when you are done.
Tip the chair onto its back and repeat the weaving and tying process, this time forming the chair's backrest.