There are two basic ways to grip for the X-choke. For both methods, your first hand goes in as deep as possible, palm up. Make sure that your knuckles are grabbing as close to the tag at the back of the gi as possible (or where the tag should be: dead center). Leave your thumb out so that you can make a fist. The "classic" method is shown at the end of this video, where the second hand comes over the top, palm down. If you can insert your thumb into the collar, close to the other hand, go for it. If not, grabbing the fabric over the neck is going to work nearly as well. If your partner is making this impossible, you can use the first hand's grip to lift your partner's chin so that you can insert your second hand under their chin, making a palm up grip. We'll refer to the "classic" method for the remainder of this tutorial, but that alternate method also works, and should not be discounted.