I think the old classic of offering her 10k runescape gp should do it.
I really think this is the sort of thing that should be carefully tailored to the person. Tying it to something important to both of you can't go wrong. A favourite place, a memory, your dick.
I'm partial to the last one but you do you. Asking folks on the internet would not be my first suggestion tbh
Really I don't think anyone can tell you what's right for the two of you. Second to last girl I dated would have loved a grand romantic gesture like in a movie but the most recent one likely would have beaten me with my own penis for trying.
Anyway that's my 2 cents. I've never proposed but I've thought about it a bit though. I wish you good luck with it, if things work out you can celebrate with vodka, and if they don't there's still vodka