I need to do this to my garage roof as its leaking. I looked on Wickes website and it gives you a how to on this (amongst other things) and with their felt you lay an underlay so i assume its the same for all felts.
Have you had a look at the DIY Doctor projects section: https://www.diydoctor.org.uk/projects.htm There are several projects on flat roofing that may be of some use to you.