Use the correct needle
Choose a stretch needle rather than a universal, jersey or ball point for the best results
Select a suitable stitch
Your sewing machine will probably have at least one suitable stitch for sewing stretch but a narrow zig-zag works - if it doesn't check the manual.
Use a twin needle
A twin needle provides a really good hem finish as it looks like a factory-produced ‘cover stitch’.
Tear away stabiliser, placed under your seam, stops a stretchy fabric from ‘rippling’ or ‘waving’.
Good quality thread
Choose a good quality, polyester thread like Gutermann to sew stretch fabric. It gives a little so works well as the fabric moves.
If you have an overlocker, use it. The feed dogs on an overlocker make it easier to sew stretch fabric and give a better finish.