A window will come up where you can enter the name of an alias and hit enter.

If the alias hasn't already been taken, it's yours.

When you go to make a post, you will see a small arrow near your normal handle.  Clicking on this will drop down a menu of all of your aliases and you can then choose from among them.

Remember:  You can not use an alias on a thread where you already posted under your normal handle, or vice versa.  Same goes for using more than one alias in a thread.

Have fun!

(PS, I still have that photo of your oily tits that you sent me!  8o)