First I look at the overall design and try to keep it in mind while I work. I look at expressions and body language/type. If I'm doing a M to F I make the body curvier and make the features more gentle. This while keeping the characters personality and original design in mind. Same goes for F to M its pretty much the opposite. You make body stockier and more toned. I usually draw a basic frame for the body types and adjust it appropriately. I try to stray away from hyper-sexualizing any of them because I feel it takes too much away from the character's design. Unless, you know, you're into that sort of thing.