I need basic direction for fixing a 3/4" steel galvanized hot-water pipe
which developed a leak right in the middle of a 50' horizontal run
(probably put there in the 1960s' in the crawl space).

I saw temporary tape and putty patches but I'd rather replace the section
that is leaking.

I'm sure I cut the bad section out and replace with a new pipe but what I
don't understand is how to make the connection.

I mean, one side will be easy but then as I try to screw in the other side,
it will unscrew the first side.

How does one replace a section of galvanized steel pipe?