Making custom enemies?

So, I'm a newcomer to the forums and I'm working on my new Doom II WAD called No Remorse. I want to create an edit of the Demon and use custom sprites and some action functions. Any way to do this? I have seen other posts about this and none of them show any step by step guide or anything... But seriously though, please help me with this...

You are in luck.
Read through this recent thread about the merits of DEHACKED versus DECORATE, where another newcomer tries to come to grips with the very same subject.

Then read up on DECORATE

If you need examples, then examine some Actors from the REALM667 Repository with Slade3.