I purchased an old crocheted tablecloth.

You can buy "Robin Linen Starch" Powder. or you can use a Spray Starch, to do as you go.

I don't think a full crocheted tablecloth is intended to be starched. What size is this cloth? You must launder this very carefully as the little interstices rip easily. It is possible to buy starch in a liquid form in a bottle. This starch is meant to be diluted.

It's called blocking. If it's not gigantic use your floor to pin the thing to size, I found it too difficult to pin it when wet so I did it dry and sprayed it wet...or take it to your dry clean place and let them do it

I have several bedspreads & tablecloths my grandmother made. For 60 years, I have washed them on delicate. Using a large surface like a table, put down towels to cover table, fold crocheted piece in half. Shape edges to be even. Fold again if it hangs over the edges to keep from stretching after shaping. Let dry. If you want it stiffer, spray it with spray starch after shaping. The only pieces I ever starched were the tatted snowflake ornaments or “doilies“ she made. I found that if shaped & dried I didn’t really need to starch thE larger pieces. If a lot of threads are breaking, (they can be repaired), just soak In tub and let drain until not dripping. The new front load washers works well since they don’t have an agitator.

Emily is right that when wet, a crocheted item the size of a tablecloth will be not only very heavy, but very fragile, and can easily damage itself.

I am wondering for what purpose do you want to starch it?

I have several tablecloths approximately that size. Sometimes it is hard to tell the difference between a tablecloth and a bed spread. But it does sound more like a tablecloth. I would not recommend starching it. And be very careful washing it and dry it flat on a bed or the floor. You can smooth it out then and shape it too.

p.s. I have a friend who crocheted these. I will call her tomorrow but may not get back to you till next day.