This is how one casual rakudo without much development experience has gone about hacking on rakudo to fix/enhance stuff. Others may have better advice.
After fighting a while with developing from inside a rakudo tree (details too long ago to remember) I found that nowadays the full-build time is short enough that it was less frustrating to just develop inside rakudobrews repository.
Once built/installed from rakudobrew the subdirectory git_reference/rakudo has a clone of rakudo. I edit that and then:
cd rakudobrew/git_reference/rakudo
git pull # if you do not this now, might get a merge commit when building
git add # any added files
git commit -a
# cut the hash
Since I do not push from this clone, I feel free to be casual about commit comments here -- they are for my own use only.
Note that if you pull, you are combining your patch with recent upstream changes. It's actually hard not to, and probably a good idea to do so anyway so haven't tried to figure a way around this. Note that if you let the pull happen during build, because you are building from the hash of your head, the new changes will not be rolled in unti your next 'commit -a' even though they are pulled into git_reference/rakudo.
Now we build the commit that has your patch. Personally I use triple in case I want to set the moar and nqp versions.
cd ../..
rm -rf moar-one_or_more_old_builds_because_they_accrue
./bin/rakudobrew triple <hash_from_last_section>
(...actually I have a heavily patched rakudobrew that allows me to use "--configure-opts=--prefix=/home/username/.local" which means my rakudobrew is usually a little stale. Hopefully tooling will catch up)
After checking via the CLI that my changes are working, running test suites.
cd moar-hash_of_this_build
make spectest
cd t/spec
git checkout 6.c # even more importantly, check that 6.c tests work
cd ../..
make spectest
These days the first one is usually clean and the second one may have a few failures that do not involve your subject matter -- hopefully that situation will get cleaned up soon. If you develop patches frequently enough you get used too what to expect to fail, if not ask on IRC for someone to gist the current failures. Or do a clean build without your patch.