Hi Charlie,
Thanks for the props, glad you are enjoying the videos 😀 I didn't cover beat mapping in them, as it is beyond the scope of an Explained title. I already went more than two hours over the targeted run time!😉
What you need to do isn't difficult. Load up a MIDI sound (hi hat or rim shot work well for this) and record yourself tapping along in time to the music. Quarter notes usually work fine; but it's dependant on the nature of the music of course. One you have that recorded and edited so that it sounds right, in terms of sounding in time with the music, open up your Global Tracks. Make sure Beat Mapping is displayed in them.
Select your MIDI region containing the correct timing of the intended quarter notes, and click hold on the Beat Mapping field. In the pop up menu, choose the top item called "Beats From Regions". This will create a tempo map following those intended quarter notes (or whatever subdivision you use). There will be a tempo event created at each sub division.
One tip: Try and line up your very first note in both your guide region and your actual regions containing the music, so that the very first downbeat lines up exactly at Bar 1 1 1 1. This little bit of extra insurance will really help the whole process work smoothly and leave you with a solid and accurate tempo map.