On a new build, the recommendation I got from some unremembered place was to NOT cut the fret wire before insertion. Lay the wire on the slot, using the "long end" as a handle, and use a wood block or soft (and very light) hammer to press in. Cut the wire after it's seated. Trim/file/grind/hone/shape/polish after you've got all the frets in. It's a bit like handling an unfriendly porcupine until you get all the ends trimmed, but it's easier than handling individual frets pre-cut to fit.

On a re-fret, if you need to do it without taking the trim off, of course you have to fit them before putting them in.

Fret wire is often sold in coils, and the hardest part (personal opinion) is getting the wire straight before you start installing and cutting. There are machines to do the straightening that anyone who does a lot of fret work would want; but us po' folk have to just do the best we can with imagination and perspiration.

John