Everyone else has said it but Roll while you’re trying them. Trying them on grass, or holding onto something is harder, you don’t keep your shoulders parallel and you end up having to learn them again once you try them rolling.
Relax your front foot as you flick it towards 11 o’clock (or 1 o’clock if you’re goofy), and allow your foot to roll, so that your shoe ‘upper’ slides up the griptape. Hard to explain but I’m sure there will be some slow motion vids on YouTube.
When I learnt kickflips, I didn’t understand the parallel shoulders thing and ended up learning them with a body varial (or a sex change as they were called in the olden days). Took ages to learn them properly. Didn’t have this issue with switch flips, as parallel shoulders feel more natural when skating switch.
The main thing is, to just practice, practice, practice. You’re spinning a bit of wood with four wheels around and trying to land back on it. It’s not going to be easy 
