They are easy to understand, but they do not sound natural. Wait until you have been here a few months, then decide. (You do not need to use "it" at the end.) Stay here a few months, then voice your opinion. ("Say" and "voice" mean the same thing, but we would say you "voice your opinion" instead of you "say your opinion.") Let me know if you'd like any more help!

Wait for a few months, then decide it. Stay here for a few months, then say your opinion.

These sentences are both fine, however, they aren't very natural. We'd understand what you are trying to say, however. It'd be more natural to say: "Wait until you've been here for a couple of months before making your decision." Hope this helped! -Kyle