1. Put your Video Clip A on video track 1.
2. Put your Video Clip B on video track 2 so it overlaps with Video Clip A at the point you want the transition to occur.
3. Put the transition on video track 3 directly above where Video Clip A and Video Clip B overlap.
4. Apply a track matte key to Video Clip B. To do this go to Effects>Keying>Track Key and drag it on top of Video Clip B.
5. Go to Effects Controls>Track Matte. Matte should be set to Video 3 (since your transition is on video track 3) and Composite should be set to Alpha. *Note: If the composite setting “Alpha” does not work, try Lumos.
Here is an example of what it should look like in the end: