This is a clicker training video on how to teach your dog to cross their paws. If you don't have a clicker you can always use some other noise maker like a clicking pen, or just a word like "yes!"
There are many ways to teach this behavior. Another being, holding out your hand and asking the dog to "Shake" and then whipping your hand away so that the dogs foot lands on top of the other foot. There is also another way, where the trainer can lure the dog to the left or right when they are in the down position and the dog will naturally cross their paws to reach the treat, which you can use your clicker to capture.
Some dogs are easier than others! Kiko with her match stick legs (who NEVER crosses her paws hardly ever) was a challenge!
Also pay attention to the way your dogs hips are laying. If they lay with their hips to one side, then it might be easier for them to cross their paws in one direction better than the other. If they are lying like a lion then they will be able to cross both ways with the same ease.