Make sure its up the right way is usually a good start, may sound stupid but ive seen plenty put on upside down including mine hehe
at the bottom there should be three sliding red parts, two thick parts with indents every 2 centimeter or so around, then there is the smallest one at the bottom which is like a lock nut and u need to roll that down the shock before you start. (if you dont have any yellow part between the spring and the bottom part of the shock you will need to give it a few spins to get some room to move)
Once you roll that small one down the next one should do the same pretty eaisly aswell, now for the hard bit. The top one takes a bit of effort because thats were all the spring tension is. You need to use the wieird looking spanner things you get with the tool kit, the one that looks like a claw. hook the little tooth into one of the indentations and give it a good pull. im not really sure which way is up or down but youll figure it out.
might be a bit hard to understand without pictures but see how u go