Rearing up is something that I am still working on with both of my llamas. I started with both of them by using their touch command on a tennis ball. It is funny how llamas pick up different things quicker. Lacey learned to touch the tennis ball almost immediately but Harlley took quite a while to pick this up, I think he was actually fearful of it.
Once they got good at touching the tennis ball I began moving it around a lot, getting them to touch it up high, down near the ground, and having them need to chase it a bit before letting them catch up to it. Harlley has not moved past this step yet (mostly because he is a lot taller than me), but Lacey has now moved on to rearing up for the ball. I began holding the ball above her head so that she would have to raise her front feet off the ground to be able to touch it. She got frustrated over having to reach it and tried to walk away many times, so I helped her
out by raising her head up and not releasing the pressure until her feet left the ground. This helped her figure out what I wanted and she was very quickly rearing up by herself. She will now rear up with just my hand raised in the air and my voice command of "be tall". Below is a video of Lacey rearing up to touch the ball in my hand.
My next step for Harlley will be to put the tennis ball on a stick and have him rear up to touch that. This is necessary for Harlley because I am not tall enough to hold the ball where he can’t reach it.
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Tip: It is very important to only reward your llama for rearing up when you have actually asked them to do it. Otherwise your llama will start rearing up randomly hoping to be rewarded for it, which could be dangerous to both you and your llama.