Here are step by step instructions for using a track matte (also known as travel matte) for free stock footage video transition in Adobe Premiere Pro:

1. Put your Video Clip A on video track 1. 

TM Step 1 resized 600

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. 

 TM Step 2 resized 600

3. Put the transition on video track 3 directly above where Video Clip A and Video Clip B overlap. 

 TM Step 3 resized 600

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. 

TM Step 4 resized 600

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.

TM Step 5 resized 600

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Here is an example of what it should look like in the end: