To sharpen a saw you will need a triangular file. We clamp the hacksaw blade in a vice and at an angle, approximately 45 degrees, we begin the alternate sharpening of each tooth. First, we sharpen one through on one side, then we turn the blade over and sharpen the teeth on the other side. This is the most common and absolutely affordable way to sharpen saw teeth.
If you do not have such wealth as a vice, then the saw may well be held in your hand.
Perform all sharpening manipulations with gloves or protective gloves.
The work is painstaking and requires skill. But you get comfortable very quickly and will not notice how you will finish the last tooth of the saw. Now the saw is sharpened:
This is perfectly visible with the naked eye. We make sharpening checks in an affordable way by sawing off a board.
It is sawed well, the cut was cut even.
You can also use needle files for sharpening. I repeat, the work is very laborious, for plodding. I recommend getting rid of all distracting sources, so that nothing distracts from work. Of course, using a mechanized tool, the applied forces and time can be significantly reduced, but as already mentioned, this is the simplest and most affordable way to sharpen a hacksaw blade.