Ah, I see. At that age, the attention span is naturally low and everything seems exciting. I personally like to encourage that craziness as much as possible as it can help when it comes to other more daring tricks. My advice is, whatever else you do, don't discourage your dog from acting a little whacky. It will be a God-send to you in a few months time when you begin to teach tricks that require your dog to be pushy, daring, and even a little cheeky.
In the meantime, keep the training sessions very short so that the dog's mind doesn't have time to wander. Above all, have lots of fun between you. The more fun you have, the more you bond, and the more you bond, the more the dog trusts you, and the more the dog trusts you, the easier teaching new tricks will become as the brain matures.
If you insist on perfecting the crawl now, then make sure you visualise exactly what you want before you begin each session. I am teaching Ellie to crawl right now as it happens. When I did my afternoon session, I visualised that I wanted to sit six feet away and have her crawl forward to me, without her lifting her legs at all. If she didn't lift her legs, I clicked and rewarded. If she lifted her legs, I didn't and we started over.
It's up to you to be consistent and let the dog know how to succeed. As long as the reward is of high enough value, the dog will follow.
