They are a PITA, however the recipe for success seems to be to strip the outer slightly further back that you ultimately need. Then untwist and arrange the wires into the right sequence. Hold them tightly at the point they exit the sheath, and pull them nice and straight and as parallel as possible. Now cut the ends of all the wires with the crimp tool so they are all the same length, with a "square" end. Now you should be able to wiggle them into the RJ45. Check they are going into the slots without kinking, and once in, you need to push them home quite hard. Check both sides of the RJ45 to make sure you can see the wires going right to the end of the plug. Once you are happy you can, then crimp.
You don't need to do many to decide pre made leads are the way to go unless you are desperate! (fitting plugs is much harder than punching the wires down onto the back of a modular socket - one of the many reasons why fixed wiring is done to sockets and not flying leads)