1 · Check out QGIS

Clone upstream QGIS wherever you keep source trees — over ssh:

git clone git@github.com:qgis/QGIS.git ~/dev/cpp/QGIS
cd ~/dev/cpp/QGIS

If you plan to submit patches (you do — that's step 7), fork qgis/QGIS on GitHub now and add your fork as a second remote:

gh repo fork qgis/QGIS --remote=false      # or use the web UI
git remote add fork git@github.com:<you>/QGIS.git

Why keep origin = upstream?

Branches track upstream master and qgis-dev worktree add bases new worktrees on it. Your fork is only a delivery vehicle for PRs.

What you have now

A pristine upstream clone. git status is empty and — this is the promise this whole environment keeps — it will still be empty after every remaining step of this journey.

Continue to 2 · Embed qgis-dev-env.