Make a cross
Imagine that you have an exercise in front of you, and you have to choose the correct answer. Then, there are several pictures and you have to "make a cross" to say if it is true or false. This way you select the answer.
Is it correct if I express myself saying "make a cross" or "write a cross"?
Thank you.
5 Answers
Place/put/enter/mark an X into the correct square/box.

If you are referring to "checking off" answers I have only heard them referred to as x's (i griega), never crosses.

An X next to a True/False question would imply that it was incorrect, just as the RAE uses them next to their incorrect example sentences. A check mark would imply that it is correct, even though using T and F is much more common.
When an illiterate person didn't know how to sign his name, they used to "make their mark" rather than sign their name. That "mark" was commonly an X. That is why we customarily put an X before the signature line on a document.

....put an "x" in the correct square. Is that it?
Yes, that's correct.
Put an "x"
Place an "X"
Mark an "X"
Write an "X" (less common to say "write")
Usually your instructions will tell you what mark (symbol) to put in the box. It will usually say mark the correct box by entering a check mark or by entering an X into the appropriate box..
Some of the verbs could be write, enter, put, place, mark, etc. If they say check the corresponding box, they are saying put a check mark in the correct box, but if you put a X in it, it means the same thing.
I have two pictures in every sentence.
Picture 1. (It is an empty little square in the column True)
Picture 2. (It is an empty little square in the column False)
Then, I could say: put an "x" in the correct square. Is that it?