Apply a transition

1. In the Effects panel, find the transition you want to apply. You’ll need to expand the Video Transitions bin, and then expand the bin containing the transition you want to use.
2. To place a transition between two clips, drag the transition to the cut line between two clips, and release the mouse when one of the following icons appears:

   End At Cut icon

   Aligns the end of the transition to the end of the first clip.

   Center At Cut icon

   Centers the transition over the cut.

   Start At Cut icon

   Aligns the beginning of the transition to the beginning of the second clip.

   Note: As you drag in a Timeline panel, you can see the area covered by the transition outlined.

3. To place a transition on a single cut, Ctrl‑drag (Windows) or Command-drag (Mac OS) the transition into a Timeline panel. Release the mouse when you see either the End At Cut or Start At Cut icon.
   If you drag a transition to a clip that is not adjacent to another clip, you don’t need to Ctrl‑drag (Windows) or Command-drag (Mac OS). The transition automatically becomes single‑sided.
4. If a dialog box appears containing transition settings, specify options and click OK.

 community help (adobe.com)