Needles and fids
A needle turns a fraying end into a tool that can be pushed through a tight braid. For collar and lead work it is not optional.
Types
Screw on needles thread into the core of the cord. Fids are hollow and take the cord inside. Both do the same job; the screw type is faster, the fid is gentler on tight weaves.
Sizes
Needles are sized to the cord. A 550 needle in 425 cord will not hold, and a 425 needle in 550 cord will not screw in far enough.
Technique
Melt the cord end, roll it to a point while warm, then screw the needle on clockwise. A needle that keeps unscrewing during work was attached to an end that was not tapered.
Buying
Needles are listed here: paracord needles (https://www.123paracord.co.uk/paracord-supplies-and-accessories/paracord-needles/).