When you tell someone "___ yourself!" you're saying that they should do it, not you or someone else:
A: Clean up that mess!
B: Clean it up yourself.
If you want something done right, you have to do it yourself.
You can also use "yourself", "myself", "herself", etc. in questions and descriptions:
Are you sure that's what happened? Did you see it yourself?
A: Your hair looks great. Did you get it colored?
Notice that "___ yourself" is different from "___ by yourself". Doing something "by yourself" means doing it without other people helping or joining you:
Sometimes I like to go out to see a movie by myself.
Eating lunch by yourself in middle school or high school is really sad and embarrassing.
I recorded the album myself, but I didn't do it completely by myself. I got some help from Ronnie and a friend of his who's worked at a recording studio.