Get a small pair of needle nose pliers.
Set the clip so it touches the part its being clipped to. Make sure its lined up with the groove its going into. Place one part of the pliers on the clip and the other on the opposite side of the part its going on to and gently squeeze.
You can also use the end of a flathead screw driver to push the clip on.
Fact:
There are 2 sides to an e-clip. Don't put it on backward.
E-clips are stamped out of a sheet of metal. One side of the clip will have smooth rounded edges and the other side will be sharp on the edges. Place the smooth edge against the moving part.
If you put them on wrong they have a better chance of coming off.