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which means i cannot fuck this up
This is the best way to actually fuck it up.
Just be as you are normally. Girls are attracted to man's personality, if you play too many games with her you might fuck it up because you will be concentrating too much on "what's my next step", thus getting into your head too much and thus coming off as weird or wussy or whatnot.
In warm approach I'm usually more direct than in cold approach. I often say that I like the girl as the very first thing after we meet. It takes so much pressure off of me that I can "be myself" and not really care anymore how she perceives me. Works almost always.