I'm assuming you're clicker training, so forgive me if you arent. It's very simple to add hand signals.
Once the dog is doing the command consistently, every time you are in a clicker session, or if you have already added a verbal command, every time you ask for it, you start to add the hand signal. (or slowly raise up, or change how you lured the dog into position and make that the hand signal).
Right as the dog starts to do the action you add the signal, or in conjunction with the command word if you've alreayd taught one.
So Example of this is, my dog is learning to crawl right now. I started off luring her across the floor from a down, to the best of my ability (she almost never lures for me) by putting a treat in my palm and then wiggling my fingers to get her to notice it and eventually follow it. I added the command "crawl" every time. I am now working on getting her to crawl without my hand being flat on the floor, and eventually I'll be able to wiggle my fingers, palm side up, any time and at any level and she'll know it means "crawl towards mom."
Does that make sense? You want to add it before you click, and while the dog is in motion. If you add it before hand, as in you are using it to ask teh dog to do something, it will confuse it. If you add the signal after it's done moving, it won't know what it means either. And if you add it after you click and treat, well, the dogs focus is not longer on what it just did and on what it is now eating. It is usually very simple and doesn't take long if the dog is already doing the action you want before hand. Once you get the timing down when teaching hand signals, you'll be zipping through them in no time!