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There are a few ways to do this. From the documentation you linked:

> When the variable `$SYSTEMD_UNIT_PATH` is set, the contents of this variable overrides the unit load path.

Option 1: Edit that environment variable to something like `/home/me/custom-units:`. The trailing colon indicates the usual load paths should also be used, with a lower priority.

> Moreover, additional units might be loaded into systemd from directories not on the unit load path by creating a symlink pointing to a unit file in the directories. You can use systemctl link for this...

Option 2: Use this command. An example invocation is `systemctl link /home/me/custom-units/my-custom-unit.service`. This would link a single unit file into systemd so it can be started with `systemctl start my-custom-unit`. If there are other units in the same directory, those will need to be linked as well. Links persist until `systemctl disable` is run on them.

> Unit files are loaded from a set of paths determined during compilation

Option 3: Compile your own systemd and change the paths in [systemd.pc.in](https://github.com/systemd/systemd/blob/3473c842fa4d41d873acd076f227a80c69d44450/src/core/systemd.pc.in#L46). Not for the faint of heart.