How should I ask the girl I'm dating to be my official girlfriend?

Ok there’s this girl that really likes me. How can I tell? We’ve been dating for three months, she tells me she likes me a lot + we mess around a lot. Anyway I have developed feelings for her as well. When is a good time to talk to her and ask her to be my girl?

1) After we mess around and are cuddling. Is this an appropriate time?

2) On our next date I would basically pick her up and tell her, “I want you to come with me as my girlfriend and not just friend anymore”, possibly throwing in some flowers to add to give me some “romantic” points (never got her flowers yet).

3) Any other suggestions?