Update

It takes some practice to get this right. Here's how I do it.
If you can, have a bowel movement before using otherwise it may stimulate you to have one.

First, I use petroleum jelly as a lubricant. (It's cheap and effective.)
Put a little up into your rectum and a little on the part of the enema to be inserted.
You don't really want to bear down. Just relax. You can bend over and slowly insert the enema... squeezing gently.
You may have to ease it out a bit and squeeze again.
Wipe off the excess lube and get into bed. Stay on your left side for at least 1/2 hour.
With practice, it gets easier.
A skill you never thought you would have to perfect.

As you heal, and with practice the enema will be easier to retain. You are going to have a slight urge to evacuate the enema right away, but you must ignore that feeling and it should go away within a minute or two.

The key thing about enemas is to make sure you've squeezed all of the air out before your insert it. As if air remains, it will cause you to have to evacuate it rather quickly.

It also helps to get the enema up to body temperature before you begin, so I'll usually hold mine in my armpit for a few minutes before inserting it.