When I used to train apprentice jewelers, I would start with sawing and filing. I would give them sheets of copper and brass and tell them to cut out different shapes.Then they would file them to perfect squares,triangles,etc. of matching size. I would have them file a 1/2" dowel into a triangle,a square,a hexagon and an octagon.Some of those guys and gals still have that multi shaped pieces around to remind them how hard it was to learn filing.
Take your time filing, it is not a race against the steel.
Practice long smooth strokes, NEVER "saw" a file back and forth -one way only.
The angle is not so important as the technique. Once you have mastered filing, the angles will become intuitive.
Most of all, have fun.
Stacy