I communicate with them by holding them in my left hand and then I just talk to them. Then I wait for thoughts popping up in my mind. I accept any thought, any answer without questioning them. It wasn't easy in the beginning but after a while you will be able to distinguish their thought forms from your own.
I learned that there are crystals which easily respond, and there are crystals which never respond. Or they only respond after the x time. Sometimes they respond with images or dreams. I've got a spirit quartz that has put a song in my head for many months each time I held it
What helps is that you first ask their permission to communicate, and that you address the right question to the right crystal. I mean, it's useless to ask a quartz crystal how a malachite could help you.
Also, don't be surpised if they give funny answers, or play tricks with you. They love to do that