Flat 3 was my first invert and is a good one to start with.
Step 1. Make sure you are comfortable with flat 3s/5s on tramps. this will make the rotation feel more natural and make it much easier to land. Also being able to flat 5 on tramp helps because you may over-rotate when you try it on snow.
Step 2. Find a decent sized jump around 15-20 feet. Try to find one with a decent amount of pop. Kickier jumps are easier to flip on.
Step 3. Do some straight airs and straight 3s and try to focus on your pop off the lip. Make sure you feel your shins on the front of your boots (try to imagine your balancing a quarter in the front of your boots and you don't want it to slide down but you don't want it to hurt your shin).
Step 4. Be patient as you come off the lip. Wait until the end of the jump until you set the flatspin. feel your shins on the jump and as you pop, lift you hips up to the sky in front and to the side. DO NOT DROP YOUR SHOULDERS DOWN. Dropping your shoulder into it is called slinging and will cause you to lose all control of the trick.
Step 5. As you come around, look for the knuckle and swing your feet underneath you. Good grabs to help with the rotation are safety and japan.
Step 6. Stomp it, head up and do it again.