Attach elastic to wrong side of binding.
Before attaching the elastic stretch it out, then remeasure and trim it (it will actually grow in length a little when you stretch it out).
Give the elastic a very slight stretch when attaching it.
If using overlocker, increase stitch length to avoid bulk.
If using sewing machine, use a long wide zig zag.
With right sides facing align binding to main fabric, matching at quarter/half points as appropriate.
Attach binding to main stretching between matching points.
View of right and wrong side after binding is attached.
Wrap edge of binding to wrong side.
If you need any help with your binding, you can add photos or questions here.
HELP POST FOR BINDING