Step 1:  Find a Youtube video that you want edited.

Step 2: Using the editing tools in Google Slides, insert and trim the video, as well as  perform other editing functions such as removing audio, adding shapes to mask content, etc. 

Insert/Video:  

Next on the right hand side, make changes in Video Playback.  When you click on Play, it will only play the specific section of the video you have chosen.