It's only barely been mentioned... start by having the saw in a vice, preferably a vice with swivel base so you can spin the saw around.
Secondly, practice keeping the file angle consistent through the stroke.. I'm a southpaw, but for the right cutters I have the powerhead on my left, my right hand resting on the bar and hold the end of the file with 2 fingers, and my arm doesn't move, just my wrist.. keep a close eye on the file and you'll be able to keep it straight and level after a while.
On the left side cutters, I spin the saw so the powerhead is under my right arm, and my arm rests on it (elbow around the clutch cover), and the "feel" of doing the other side is exactly the same, both my arms are doing the exact same motions, which keeps the two sides of the chain the same..
I don't know if this can be swapped for a right hander reasonably.. Right side cutters I would do with left arm on the bar, facing the powerhead, left side I would probably be over the saw, with both arms anchored to my sides?
I gotta take a vid someday