Im a begginer at skateboarding and im having some trouble doing the ollie?

im having trouble bringing my knees to my chest after you slide your foot up i feel that i dont have enough time in the air and when you do that do u jump or just bring ur knees up. also when i land i always find my back foot moves forward off the tail and my front foot will be half way off the side of the board not over the bolts this makes it difficult to land and it looks really weird when i land what should i do, thanks :)