Since she has not been taught to stack from an early age, it will take much more work. Start by teaching her to sit. Once she masters it, you need to teach her stay. Then you can move onto a stack.
You start by teaching her to stay in a standing position, then you give a release command, such as "okay." When she starts standing and waiting after you instruct her to, you then start to pull out her rear leg and place it on the ground in the proper position. Hold it there and continue to tell her stay. Then give your release command and give lots of praise.
She will get it eventually. It just takes time and patience =D