You can't take the ends off and re use them. They metal pins lock into the end of the wire internally.
They of course sell new connectors that you can crimp on. Crimping the ends on cables is one of those things you only learn by doing and you will make a number of bad ones so you do not want to use your actual cable to learn on unless you have enough excess you can keep cutting off the bad ends.
In many cases it will be cheaper to find prebuilt cables because of the tools you must buy and the wasted ends you have learning, but if they won't fit though some hole then you have no other choice.
Depending on your application you might look at keystone jacks especially if you can mount them in the wall. They make toolless versions of these if you look around. They are easier for beginners both because they print the wire colors on the jack and you can do 1 wire at time rather than all 8. It is also pretty easy to try mulitple times if you mess up.