Hello, sorry but I was very busy during the last days... I don't know if it needs to make a tutorial on this subject, because you can see this action in a lot of my videos... Its very easy... if you have the transition file (which you will use as matte) you do like this...place your first video (or image ) at the bottom, place the second video above it, an make the layer start from the moment you want the transition to start, then place the transition layer on top of them (and make it start from the same point as the second video) then change the track matte of the second video to alpha matte (if you don't see the track matte option in your panel, press F4...)
I hope this will help you.