Ask yourself: “Why do I need custom humanoids?”
What is the project? How will Humanoid NPCs drain performance? Are custom Humanoids going to be better?
I recommend staying with humanoids unless you don’t plan to use any of their useful features, which is doubtful. You can still optimize the default humanoids significantly:
https://devforum.roblox.com/t/humanoid-internal-hindrance/20057
Otherwise, this is a really vague question. There’s no single way to make your own custom “humanoid,” and as such the approach you take will be different depending on the situation. Hopefully this helps. 