How to make signs stand

An easel is usually used in that case!

Agree with the previous comment - usually an easel will help hold it up! You can find those for fairly cheap at an art store or online. Otherwise, if it's sitting on a table or something, you can prop it up against a wall or something else.

Buy or rent an easel.

Find out if you can rent an easel.

Agree. I would also ask your venue if they have an easel you can use before you rent or buy one. Sometimes, the venue will throw that in if you need it. My venue let us use 2 for my wedding.

As others said, an easel (you can find them pretty cheap online and you could paint an inexpensive wooden one) or put the sign in a frame that is self-standing and put it on a table.