Open the After Effects program. You can get a free 30-day trial from Adobe if you don't have it already. It comes with everything you need.
Import your footage to the Project window. This should be the farthest left-hand window. At the bottom of this window you should see a few icons: a folder, a film strip, as well as others. Look for these then move onto the next step.
Create a composition before even trying to add slow motion. Drag your footage from the project window to the small filmstrip icon next to the binoculars icon and folder icon at the bottom of the project window. This allows your composition to automatically become the same time and size of the movie you have.
Select your layer and click Layer, then Time, then Enable Time Remapping.
Look for two key frames in the shape of yellow diamonds below your movie. These control the speed of your film. Bringing them closer makes time go faster and pulling them apart makes them go slower. Hence the slow motion.
Add more time to the composition to apply slow motion to the whole clip. Go to Composition, then click Composition settings. Go down to Duration and add more time to the composition. Move the last yellow diamond key frame, and your whole clip will be in slow motion.
Select only part of your clip if you do not want the whole composition to be in slow motion. Go to the part you want in slow motion. Add a key frame by clicking on the empty diamond to the left of the Time-remapping effect title under your film. Set where you want the slow motion effect to stop. Pull these two key frames apart and then you have slowed down time for a portion of your film.
Move your clip into your project window, then click on it. There should be a small blue icon at the bottom.
Click on the clip, then go to Clip Adjustments, then click Speed. This will allow you to make the clip faster or slower.
Look for the part of your film that you want to appear in slow motion, then cut the clip and add the effect to it.