Good instructions @Simone!
Here I would use the following command instead, because it will do a better job of resetting and cleaning up your local branch so that it doesn’t include any unwanted changes:
git checkout upstream/master -B master
After that, the git merge step is unnecessary (it should do nothing). In step 9 you don’t have to “hope the merge has gone right” either, since there was no merge.
Note that for some repositories (including ClassicPress) the default branch is develop rather than master, so you’d use develop everywhere the above instructions say master.