I agree. It’s very challenging to get the pairs in the connector correctly. I tried different tools, made no difference….
I guess some people are just good at it and some aren't. I am good at it, but it does take me some time to get it done. Here's how I do it (I never had a class or anything, just figured this out on my own, so there may be some unusual steps in my method):
Strip the outer insulation, un-twist all the wires, straighten them out and put them in order (know the order forwards and backwards so that you can look at it from either side while working). Then pull them all out straight, bend the wires back and forth a couple of times to get the twists and kinks out, you want them to be perfectly straight before you attempt to put on the connector. If they don't straighten out easily, use a needlenose pliers on each individual wire if necessary. That's the time-consuming part. Then, put the straightened wires all back together in order, verify that the straight part is long enough to get to the end of the connector, and cut the ends all off evenly so that they're just barely long enough because if too long, the connector won't crimp down on the outer insulation. Then, put the ends in the connector, usually I do it with the tab facing me so that I can see better if they go all the way forward. If not, I wiggle and push some more until all 8 wires are all the way to the end of the connector. If one of the pairs absolutely refuses to go all the way, you can also pull it out, and re-straighten and pull on the wires again and then see if it'll go in. If it gets almost all the way, within two millimeters, the connection will probably still work, but if it's more than one millimeter I usually start over and re-do it. At that point, just before crimping, I triple-check the order of the wires (check it again at least twice more after the wires are in the connector but before crimping). Then crimp it two or three times to get it good and tight. If you have a more expensive crimping tool the tightness is more guaranteed to be correct the first time.
Then I don't even test it because I know I did it correctly.