The first step: Start with your weight centered over your back foot, so your head, chest and hips are all directly above your back foot. You need to keep your knees and hips loose. Shift your hips towards your front foot; your upper body will start to follow. You have now transitioned your weight to directly over your front foot. On a shortboard, this would be enough! You now won’t have any weight or pressure on your back foot.
The cross-step, part 1: With your weight over your front foot and your back foot unweighted, you can bring your back leg IN FRONT of your leading leg, keeping your back-leg knee towards the tail of the board. If your knee goes over the top or in front then you will loose control over the weight transfer process. Your back foot, which is now the closes to the nose, will probably land on the outstep rather than flat. As this feeling becomes familiar you can “lock” your knees into each other and surf in this cross-legged position without any problem. Because you haven’t yet transferred any weight to your new front foot you can easily step-back should you need to.
The cross-step, part 2: (the transition): In this cross-legged position, you can now shift your hips forward (again, your upper body will follow)so that they are over your new leading foot.
The cross-step, part 3: Now that your weight has transitioned and is planted over your new front foot, if required and/or desired you can now step your new back foot (which is the leg that you naturally lead with) through, behind, to return to an open (uncrossed) stance. This new leading foot doesn’t need to have any weight applied to it, if you’re up near the nose of your board, and can easily be returned to its original position if you need to move back to adjust trim or slow down. If there is still scope for moving further forwards, then you can again shift your hips forward to move your weight further along the board.