Fission is the process where nuclei:
Split to form smaller nuclei.
Fusion is the process where nuclei:
Combine to form larger nuclei.
A easy way to remember which is which is by remembering that to fuse two things together is to combine them to make a single entity, and fusion is the process of fusing nuclei together. And if you know that, then you will know that fission is the process of splitting nuclei.
Fusion: Combine
Fission: Split
I hope this helps. Let me know if you have any questions or if anything confuses you. :)