How to slow down one part of a video

just cut out the part you want slowed down and then change the speed.
to cut out the clip press "c" on your keyboard to get the razor tool and click on your clip in the timeline where you want the slow motion to start and where you want it to end. then press "v" on your keyboard to get back to your regular mouse/selection tool

^dont do that. It will look dumb. keyframe that shit. youtube is your friend. its pretty easy.

Either keyframe or cut the part you want slowed down. Cutting it in a certain spot does the same thing as keyframing the speed unless you take it a step further and mess with the tabs on the keyframe then you can decide when you want the slow down to start occurring and makes it look a little smoother. If you don't care about it just cut up the clip and slow that one part down, to me both look fine as long as it is intentional and doesn't seem out of place.