A few words of advice:
1): Do not strip the outer jacket more than what is in the directions.
2): Do not untwist the wire pairs (as little as needed)
3): Be sure the ends of the wires hit the end of the inside of the connector.
4): A ratcheting crimper is best to be sure of proper crimping
5): Be sure you buy connectors that are designed for the type of cable you have.
The connectors for Solid Wires are not the same as for Stranded Wires
http://www.cableorganizer.com/articles/difference-between-solid-stranded-rj45-plugs.html
How do You Identify a Solid or Stranded RJ45 Plug?
So let's say you've got yourself a connector and aren't sure if it's stranded or solid? You may be wondering, is there some way to confirm this visually? Why yes, there is, thanks for hypothetically asking.
We really must stress that the best way to ensure a correct connection is to know which type of RJ45 and cable you're dealing with beforehand. However, if you've got an RJ45 and you're not sure if it's solid or stranded, there are some methods that could help you figure it out. Here's how it's done: if you look at the plug from the side, you'll see the tiny metal connector pins whose job it is to pierce the cable jacket and contact the copper wire. On these pins you'll see little “teeth” that do the poking and then contact the metal in order to make a conductive connection. Typically (and we must emphasize that word) a solid only plug will have longer, staggered teeth, and they will be more widely spaced from each other, since they are only focused on straddling a single, relatively thick wire.
A stranded connector, on the other hand, may have more numerous teeth, and they may be spaced closer together, since their job is to get all up in between the stranded filaments. If the teeth are spaced, the space between is usually shallower. Their job isn't really to straddle the filaments, since piercing the jacket and getting between the strands will make the necessary connection.
And that's really all there is to it. Hopefully now you find yourself a little better equipped to identify your RJ45s, and determine which one you should be using.