jackdonovanbump working on learning running gainers and flatspins
There is a couple different ways for gainers, both require strong legs and a good vertical. Full forward momentum or with a j-step.
The jump is sort of like a lay up in basketball, one knee comes up(imagine uppercutting someone with your knee) as you jump UP off of your dominant leg. Your arms should swing up in front of you as you are jumping and should be about horizonal as you leave the ground(they will help a lot, especially when you move onto corks). once you jump bring your other knee up to your chest and look back, you will be in a tucked position and go around if you commit, keep your chest up otherwise you will come around and land all bent over ( i still have a lot of trouble there)
Start practicing them with a j-step, instead of walking directly forward, arc a wee bit and as you plant your jumping leg, angle your foot outward a little bit... i usually jump off of my left foot so i put it down so my big toe would be facing about 10 o'clock instead of 12. That movement really helps a lot and is the foundation for a lot of tricking. As you get comfortable with those then start arcing less on the approach and taking off without a jstep.
They are not as bad as people work them up to be, similar to a standing back tuck... if you can jump high enough then you are physically capable of gainers and more, it is mostly just a mental game.
here is some videos to geek out on, i used to watch these guys all the time.
mr. double abcd
https://www.newschoolers.com/videos/watch/776226/Mr-Double-ABCDEF---Atlantis-Within--2011----TRICKING
vellu
https://www.newschoolers.com/videos/watch/776227/Vellu---Tricking-Showreel-2014
both of those guys are more on the tricking side of movement. The flow show used to be the best thing ever when it was around, there is some amazing talent featured and lots and lots and lots of tutorials and just all around positive stoke.
https://www.youtube.com/user/flow