FAQ

Create a subrepo from a folder in old/existing work, for use in other projects (i.e. split from git subtree)

First cd into your local copy of that work.

Now we create a subrepo of that work, then create a branch for that subrepo:

$ git subrepo init <subdir>
$ git subrepo branch <subdir>
$ git branch
* master
  subrepo/<subdir>    <--This is the branch containing <subdir> and all its history

That branch can be pushed/pulled wherever you need.

Instead of putting <subdir> into its own branch as shown above, you may put the <subdir> into a remote repo/branch:

$ git subrepo init <subdir> [-r <remote>] [-b <branch>]
$ git subrepo push <subdir>

Do whichever makes the most sense for your <subdir>.

To push changes you've made back to that old project, you may either use git push, git subrepo push, git subrepo pull depending on the context.