Apply Time Stretch to your clip.
The easiest way to do this is to right-click on the clip on your timeline, open the Time Stretch tool and set the playback speed.
Have you had a look at my free Basic Training tutorials at Muvipix.com?
You can use Time Stretch on the Clip. Now, if you have a longer Clip, but only want this applied to a portion of it, just use the Scissors/Razor to cut that longer Clip, where you want the Slo-mo to start and again, where you want it to end. Use the Time Stretch Tool to extend the desired Clip.
You might want to also look into the Effect>Posterize Time. Some like its effect better, but only your eye, and the footage, will determine if that works well, or poorly.
Here's the text on Changing a Clip's Speed:
Note: Steve beat me again, and by about the same differential. Gotta' get a T-1 line, and learn to type faster, if I plan on beating him.
Change a clip’s speed
Changing clip speed changes the clip’s duration. Speeding up a clip removes frames, thus shortening the clip duration. In the same way, slowing down a clip repeats frames and thus adds to the length of a clip. For audio clips, a change in speed also changes pitch. The Time Stretch command includes an option to maintain the original pitch of an audio clip at any speed.Note: When you change the speed of a clip containing interlaced fields, you may need to adjust how Adobe Premiere Elements processes the fields, especially when the speed drops below 100% of the original speed.
- If you want to slow down the clip, first drag it in the Timeline to the end of the movie or to another track with empty space, so you can stretch it without bumping into an adjacent clip. When slowing down a clip with the Time Stretch tool, you cannot stretch it past the edge of an adjacent clip.
- Click the Time Stretch tool at the top of the Timeline.
- Position the pointer over the edge of the clip you want to change until the correct icon appears:
Time Stretch In icon to time stretch the clip to the right of the pointer.
Time Stretch Out icon to time stretch the clip to the left of the pointer.
- Drag the edge of the clip, increasing its length to slow it down, or shortening it to speed it up.View full size graphicChanging clip speed by using the Time Stretch tool
- Preview your changes, and then make adjustments as necessary.
- If you moved the clip in step 1, click the Selection tool, and then drag the clip back into place in the movie.
- If you want to slow down a clip that has another clip on its right in the Timeline, drag it to an empty track or to the end of the movie so you can stretch it without bumping into an adjacent clip.
- Select the clip in the Timeline. Note: If you haven’t yet inserted the clip into the Timeline, you can select it in Project view of the Tasks panel instead.
- Choose Clip > Time Stretch.
- In the Time Stretch dialog box, type a percentage for Speed. A value less the 100% slows down the clip; a value greater than 100% increases its speed.
- (Optional) To keep the pitch of an audio clip unchanged, select Maintain Audio Pitch.
- Click OK. Preview your changes, and then make adjustments as necessary.
- If you moved the clip in step 1, drag it back into place in the movie.
Though he beats me to the punch all of the time, I strongly second that Learning Series, plus all of his books on PrE (and PSE and DVD Architect).