Approach the box, rail, ledge whatever in an ollie position.  Ollie onto it, just like an ollie 50-50.  Ok, here's the tricky part, depending on the lenght of the object it can be difficult to replace your feet in kickflip position before the end.  You can try and jump a little bit into the flip position, and at the end just pull a normal kickflip.  Ride away clean