This techniques explains how to put elastic in a casing – this is very useful in your knitting and elsewhere.
Put elastic in casing
Use safety pin pinned to one end of elastic. Pin loose end to garment to avoid pulling it into casing. Thread elastic through casing, evenly gathering and coming out at the beginning. Use sewing thread to fasten the two ends of the elastic into a circle. Release elastic into casing and smooth gathers. Close casing with yarn if desired.