Put in two or three screws on one end (if 2 put them opposite, if 3 equally space them around the truck*) tighten each one a little bit so you've still got a gap but the threads are holding properly, put the other end on and do the same.
Now start to tighten the screws up, one at a time just a little bit until you've got it on tight, do the other end and then put the remaining screws in.
* Officially use 3