Files are one bit if kit that seem to get over looked in the workshop, they have been around for many years, I read that Crewe loco work went through a tonne of files a month in its early days, it being two mens work for a week to file up a cast connecting rod.
I can't remember being taught how to file it was just something you did, but I do remember when I started my apprenticeship the fist task we were given was to file up a block of steel flat and square then file a square hole in the middle to fit a block, boy were we sick of that block by the time it was finished.
Any way here's a few book tips about files.
1:- Keep new files for softer materials as they become worn start to use them on harder materials.
2:- Don't let them rub together this will damage the cutting edge.
3:- Keep the file clean of bits sticking in the teeth with a stiff wire brush.
4:- Rubbing chalk on the file will help prevent soft material sticking in the teeth (Pinning)
5:- Start with a rough cut file then finish of with a fine cut or smooth file.
6:- Always use a file with a handle the tang can stab into your hand with bloody results
7:- Fit the handle nice a square so that it is comfortable to use.
8:- Have the work at elbow height
9:- Alternate direction of filing this way you can see where you're taking the meat from.
They come in different degrease of rough nes the roughest are called Bastards (school boy giggles) then we have 2nd cut smooth and fine.
Files come in all shapes a sizes

These are flat files the parallel ones are called hand files and have a safe edge with no teeth so that you can work up to a corner, in fact a lot of my files were passed onto me from engineers long gone and many of them have been modified for some job or other by having sections of the teeth ground away.

Halve round, triangular or three square, round or rat tailed, and square files

Some unusual shaped files the ones on the left have a cross section like a knife Blade, some are double radiused with a cross section similar to an aircraft wing. The bent ones are called rifler files

Needle files or Swiss files (the Swiss call them English files)

I use wooden handles but you can also get plastic handles. The correct way to fit a wooden handle is to heat the tang of the file up to a cherry red and burn the file into the handle keeping it square.

Hope that's of use to you
Stew