Asking a girl to be your girlfriend can be very awkward.  It doesn’t even matter if you know how she feels, it is still a weird situation.  To make things easier try the following things:

Figure out what you like about her.  Make sure that you really want to take this step in your relationship.  Think about what will change and if that is something you want.

Have a clear idea of what it means for her to be your girlfriend.  Know what you expect to happen after you make it officially.

Be ready to clearly explain to her why you want her to be your girlfriend.  Have a good reason and be sure you can clearly communicate it.

Choose the right time and place. Make sure you can be alone and not interrupted.  Don’t spring it on her either.  Let her know that you need to talk.  Just tell her you want to discuss something with her and make sure not to make it seem bad or she may jump to the conclusion that you are dumping her.

After you ask her, let her talk.  If she needs time to think then give it to her.  Be sure you are being sensitive to her feelings.

Take these steps and things should go rather smoothly.  If you already know that she cares about you then chances are she will be excited about being your girlfriend and the evening will probably end on a high note.