How to make home make elastic table cover?

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I've either sewn bias tape all the way around on the wrong side, then fed the elastic through using a large safety pin on the elastic end. Or if the fabric isn't too stiff, just turn under and make a channel. I leave about 3" to fold under, elastic has to be smaller than table circumference to hold tight.

for a rectangular table use fitted twin sheets.

I was going to say try a fitted sheet too. You can also use a flat sheet and cut to size you need. Leaving plenty of overhang, make a casing and thread elastic thru with safety pin as your shuttle. Make sure you secure the other end so you don't accidentally pull it through. (Spoken from experience 🙂)