How does a person jack off like step by step and how is cum build up

There is no one way to masturbate and people like to do it differently. Just do what feels right. But I'm sure some guys here will be happy to tell you what they do.

Your body make sperm constantly- every single second of every single day. That is part of the reason you have a prostate gland because it manufactures and stores sperm.

We could, but the best way is to visit Jackinworld.com
Here every imaginable way is discussed in detail.
(Remember, you are on your own as some of these could be downright dangerous or messy.... so be careful.) Other than this, just do what feels good behind locked doors and remember cleanliness.
Don't forget to come up for air and do your school homework.