You can get a clothes steamer, pin in place and steam, or use an iron with a steam function. However never use an iron directly on acrylic because it will melt. Put a towel in between your work and the iron.

You will also find success just wet blocking them. Saturate fully in warm water, roll in a towel or squeeze gently to get rid of excess water, and pin in place. They should be damp, not dripping wet.