Watch this instructional video to teach a pet rat to walk on its back legs. When trying this, don't spend too long training the rat each day, 5 minutes a day is long enough and even at that they are clever enough to pick this up in a few days. This is important too to make sure they are not over-fed during training. Also, don't be in too much of a hurry to increase the step count, take time to get the rats used to the trick and it will make increasing the steps later easier and more stable. Best to basically let your rat dictate how quickly they learn it.
Finally, I'm really not overly happy with this video so i may "re-do" it at some stage, so apologies if its not helpful. More coming soon.