Position your front foot near the middle and your back foot on the tail – The best position is for the ball of your back foot to be slightly hanging off, and the front foot more towards the middle. The position of the front foot can change, however, depending on how much air you want to get: if you’re planning on doing a monster jump, move the front foot closer to the back; if you’re just hopping a little bit, move it closer to the front of the board itself. Beware though, the closer you move it towards the back foot, the harder the trick is to perform.
It's a neat piece of science art, and it also tells us something interesting. The arrows show us that the force on the skateboard is constantly changing, both in magnitude as well as in direction. Now the force of gravity obviously isn't changing, so the reason that these force arrows are shrinking and growing and tumbling around is that the skater is changing how their feet pushes and pulls against the board. By applying a variable force that changes both in strength and direction, they're steering the board.
So with your back foot, you want to kick the tail of your board down and right when it touches the ground you jump in the air off your back foot. This is the ‘pop’. It takes a little bit of getting used to but most beginners don’t really understand that initially, your back foot is doing most of the work. Then bring your knees up to your chest after the jump, the higher up you pull your knees the higher up your skateboard can go!