In the past, I have used both git submodules and git subtrees until I eventually ran into so many problems with them that I just gave up and started looking for alternatives. I stumbled over git-subrepo (github), so this is the next thing I will try and then most likely give up on in 6 months. Since I already have trouble remembering what I need to do just to get started, here are some notes for future me.

The setup is as follows: I have a local repository and multiple external repositories that I want to install.

1. Ensure that git-subrepo is installed. Just follow the instructions here; they are actually quite decent.
2. Go into your local repository.
3. In the root directory, execute git subrepo clone <remote-url> <subdir> where <remote-url> is the url of the remote and <subdir> is the subdir that you want to clone into. Note that the tool will not create another subdirectory with the name of the subrepo that you are adding.
4. Use git subrepo pull <subdir> and git subrepo push <subdir> to pull and push.
5. Commit as usual, the tool is smart enough to figure out what goes where.

There is a more elaborate guide available here, but I somehow always find it very hard to extract these few basic steps above from that.