Hi

You need to use Keyframes and the Effects, Video Effects, Speed option.

These are the options and things you need to add

A hi res version is here.

Note:  as a habit I always use a Keyframe ( 1) to lock an effect at the beginning of the object - this restricts the occasional glitch when changing other keyframes - in this example the Speed setting is 1 x.

The second keyframe ( 2 ) is the one where the speed ramp starts and is also set at 1 x

The third keyframe ( 3 ) is the final speed, in this example 0.25 x

Note if you want to speed up the video again after the slow mo section add 2 more keyframes, the first at the point where you want the ramp up to start, it should have the same speed setting as no 3 and the last keyframe at the time you want normal speed to resume and it should be set to 1 x

Note: if you want to go slower than 0.25 x , then it is better to make 2 cuts around the section you want to slow down/speed up because you must ungroup and delete the audio.

Here is the resulting effect of the above timeline:

HTH

John