Procedure to build missing packages from sources?

I'm trying to find the process or procedures to build missing programs and packages from sources. For example, emacs is missing so I would like to build it. I can build it locally on the IPfire or I can [attempt to] cross-compile and scp it up to the IPfire.

My first question is, is there documentation that covers the topic? If so, where is it?

My second question is, if there is no documentation, then what is the procedure?

My third question is, if it is not possible, then what are the alternatives? Is there another way to get a missing program? Or should we start looking for alternative firewalls? (I like IPfire and I prefer to stay with it).

Thanks in advance.

Jeffrey Walton