I would like to know how to make a assay in order to use it on a
protein that I am trying to purify. I just want to know how to do
them.

Can you tell us more about how you intend to make the protein? If its
by cloning, you could easily add in a HIS-tag, or FLAG tag. The harder
way would be by running an electrophoresis gel.

But, assuming you have the protein purified, and you know its
sequence, and just want to know how much of it you have, there are a
number of ways to go about it

1) Ellmans Reagent
2) Mass Spectroscopy
3) ELISA (Enzyme linked Immuno Assay)

Soory, but its time to leave work now!

Good luck