How should I ask this girl to be my girlfriend?

First of all we're both 16. I know for a fact that we like each other because we both told each other that and it's obvious we like each other when we're around each other. But when we talked about being boyfriend and girlfriend before, she said that she wasn't sure if she was ready for a relationship and wanted to keep it friends. We were both hurt after that because we both liked each other. So I asked her if she'd be willing to reconsider it and she said yes. I asked her if she'd be willing to hang out with me sometime even though she didn't give me an answer yet and she said "Yeah, I'll let you know when I'm free." I was thinking about taking a walk with her to a coffee shop near her house and then walking to a park to sit down with her after. I know that she likes me and now I think that she'd be willing to be my girlfriend if I asked her. So when we're in the park I thought it would be kinda cute to go on one knee and say, "[her name], will you be my girlfriend?" just for a joke, but also seriously asking her. What do you think? Or do you think I should just wait until she gives me an answer?