opam packaging

I'm considering taking some opam packages and modifying them to my own 
ends, starting maybe next month.  I'm collecting advice here before I 
start.

Is this the right forum for questions like this?  Thay may not seem like 
beginner's questions, but I still feel very much a beginner with ocaml.

I do want to avoid confusion between mine and the official opam 
packages of the same name.  As well as have the option of having my 
version of the package replace the official one on some of my own 
development contexts.

Perhaps all this is clearly discussed in documentatino somewhere.  If so 
I'd appreciate pointers to said documentation.

(1) Where do I find the actual packages -- source and metadata in the 
.opam directory?  Presumably once I have this, it will be clearer hos to 
proceed.

(2) What naming conventions are there to distinguish different versions 
of a package so that specific ones are picked up by programs like 
ocamlfind and ocamlbuild?

(3) Once I do have package source code and have edited it, what do I 
have to do to build the package from it?  How do I register it with opam 
(locally, I'm  long way from submitting anything to the public shared 
package library)?

(4) Do I even have to build the package and register it with opam to 
start using it?

-- hendrik