If you have a personal fork of the slate-hlm repository (e.g. created automatically by the web GUI), add it as a remote to the submodule data/libraries/hlm.
Open the workspace Slate.code-workspace in Visual Studio Code.
Alternatively,
either open a .slate file under data/libraries/hlm,
or press Ctrl+Shift+P to execute a command, and enter "Slate: Show GUI".
To let Slate adapt references when moving files or directories, consider increasing the files.participants.timeout setting in Visual Studio Code.