Files are essential when doing metal work, fabricating and mods.
They are worth far more than they are sold for.
You have immense amounts of control for fine tuning whatever it is you're building. ¹⁾

Collection of Files 2)

Draw filing is a technique used to shape, smooth, flatten and square metal.
It's name refers to the fact of applying pressure on the draw (pull) stroke. Any type of single cut file (customarily a bastard file) can be used. ³⁾
Although, most any file with decent teeth will do.
You can start with a coarse teeth file and end with a fine teeth one as well.
The file's teeth can cut on both the forward and the backward movement of the file. ⁴⁾

You can flatten the edge by moving the file forwards and backwards along the length of the material.
You can create a flat in a round bar for easier drilling.
You can also flatten square or flat stock when bowed in the middle.

Start at one end and push forward or the other end and pull backward then repeat until flat and / or shaped as desired.
Keep the file at 90° on each stroke.
Clean the shavings from the file often to help prevent broken teeth.

Clamping the material to be filed in vise:
Clamp your material in the vise with the filing area on a flat plane (ideally horizontal to your body posture).
Use wooden filing blocks to protect the piece from the jaws.
Hold the file 90° to the piece you're working on with 2 hands (as in handlebars), one on each end.
Slide the file forwards along the length of the area you want to smooth with light to medium pressure. ⁵⁾
Slide the file backwards along the length of the work piece until you reach your starting position.
Apply the same pressure as you did on the push stroke.

Keep each stroke within the same plane each direction. Be careful not to push too hard as you can bend the file or break teeth (use cutting oil or lubricants while filing).
The tang on the file can be held in either hand and the file can be used with either side facing the work piece.

Keep track of the heat and use plenty of lubrication.

Clamping the file in a vise:
You can put a short piece of plywood or 2×4 in a vice on each side of the file and clamp it in the vise.
You can also use plastic 'soft jaws' to hold the file.
The rest is the same with forward and backward motion on your work piece.
The file has to be held tight. Clamping a file without side protection can ruin the teeth on the sides.

This technique helps achieve, control and maintain square lines, a flat surface and edges better than a grinder.

Draw filing stainless steel with a bastard file 6)

A file with clogged teeth will not cut very well.
It is essential to clean any metal shavings from it periodically with a wire brush.
A file should be cleaned every time before you put it away.
This helps prevent broken teeth and chattering.

Pull the file parallel to the teeth to remove the shavings.

Useful Links: https://www.wonkeedonkeetools.co.uk/files/what-is-draw-filing