You can't keep your tongue pressed against your lower lip or you won't be able to articulate with it.

You want to spit a smooth ribbonlike stream of air directly at the far edge of the hole so it splits in two. Trying to make the space you blow through as small as possible is a mistake - you will end up with pursed lips and the air will go everywhere. Your lips should form a slit rather than a round hole - you pull them out in a rather strange smile. The best shape varies depending on the pitch of the note, but it's never as rounded as when whistling.

YouTube is a help for this. Nina Perlove's videos are not aimed at beginners, but look at the shape her mouth makes.