What is the best way to proceed with a girl if she has a boyfriend.

Not to try to spit them up but to be the next in line so to speak

Leave girls with boyfriends alone, romantically because there is NO SUCH THING as being next in line!

Find somebody(s) single like you are for romance.

You just watch to see when they break up, and be around then,

When you say, "proceed', do you mean that she also has a romantic interest in you? Are you friends, have you discussed any possibilities down the road?

My advice to you is to not be romantically involved with anyone who is involved in a serious relationship, but, even more importantly, the immediate period after any breakup, is very rocky ground to start a new relationsohip. Either the one newly single, or the one 'waiting in line'.

Allow time for healing, and be sure that your relationship doesn't start as a rebound.