I dont have a guide, but I do have working salt states at
These are packaged for simple install - you can either download the
package, or install using qubes-task - take a look at https://qubes.3isec.org/tasks.html
The package creates a sys-mullvad qube with the Mullvad GUI and Mullvad
Browser, which can be used as a VPN proxy. A disposable template is also
created so you can launch disposables, and run the Mullvad Browser, or
create distinct Mullvad VPN connections per disposable.
I never presume to speak for the Qubes team.
When I comment in the Forum I speak for myself.