There are two methods to do double jumps, the first one is tapping W followed by the up arrow, or up arrow followed by W, the second method is to double tap W or the up arrow. This trick is very difficult and is only recommended for people going for times below a 125. Learning this trick can be difficult at first, but try every method and figure out what works best for you. (If you don't understand this just spam jump as fast as you can)

Video of Level 5 Double Jump -