Add a little bit of extra room at the end of the first video clip and at the beginning of the second video clip by clicking on the end of it and dragging it across the timeline. If there is no extra time to be added to the clips, just go to step 2.
Align the two clips next to each other in the timeline. Make sure there is no blank space between the two clips.
In the menu of the editing software, select "video transitions" to see a list of all possible transitions.
Select the transition you want, and click and drag it into the timeline between the two clips.
Select the "render" option from the menu to render the transition and make it viewable.