Good afternoon!

I just picked up a few Blue Angel Clematis (type III's) plants at the store. I have been admiring them for a long time in other's gardens but have never really tried to plant them - up until now I mostly have done very easy perennials that require little pruning or care.

What is your advice on how to initially plant and prune these clematis? Right now they are staked up on a little plastic trellis in the plastic pots. They are roughly 12" each, the stem is very thin, and there is only one stem coming up from the soil.

My understanding is that I need to encourage growth of new stems, but I'm unclear as to how to go about that. Like I said, I have very limited experience pruning and training.

Thank you in advance for your advice!!!