Answer:
well you can't really...
If he has already stated that he wants to marry you one day, just wait for him to ask you, you can't force him to do something which he might not be ready for.
If you've never talked about marriage, do. Find out where he stands on the subject. If he says he's really not the marrying type, and you really do want to get married, that's something to think about.
If he once said he wants to marry you some day, just wait for him to do it, let him surprise you! If it does take years for him to do so, you could bring it up again. but you can't make him pop the question. and when you think about it, if you made him do it, it wouldn't be as meaningful or romantic right?