DIY Belt Change

Apologies as I know this has been covered a few times but I thought I'd try and prepare and get as much up to date info as possible!

Looking to do the belts myself for the first time so any advice/tips/tricks I should be aware of? I've watched some videos (back in the garage) and read some threads here. Main questions at the moment are: is there a special tool to lock everything in place to stop the timing going out? Can someone explain the belt tensioning with the allen key? Saw it mentioned a few times and seems to be what most people do, I have a rough idea what it is but best to make sure