So, maybe not strictly a tooltime topic but it does relate to USING tools so I thought it was appropriate.

Anyways, doing some tinkering at home and I need to file down some aluminum surfaces to get everything toleranced. Basically, they are cylinders 19.5mm diameter x 13mm tall which I need to take 3mm off the height of. Managed to get one done pretty good using my eye and my hand on a big [censored] file and cleaning up with a ceramic sharpening stone. Had a very hard time keeping the working surface square and even. I suspect that due to imperfect pressure using my hands to hold everything it tended to file/grind/abrade unevenly. I also tried using my grinder on the flat side of the wheel which was much quicker and easier.

Any suggestions for keeping everything square? Its too small to mount up, and I can't really see a jig design that would work...

If you start off square, sandpaper on glass. I've done aluminum intakes and an oil pump housing by sliding them on sandpaper on top of a thick sheet of glass.

I've done similar using a Taig lathe.

I will wrap things with wide masking tape, and if the ends line up overlapping, then it's pretty square.

For stuff like this a surface grinder is the correct tool but most people don't have one in the garage. A small stationary belt sander with a fine belt works well too if you have one.
You could probably do it with a orbital sander by holding the piece against it and moving it around slowly, start with 180 grit until its almost there than switch to a fine grade. A pic of what you want to do would help.

If you can scribe a line that you can file or sand to it will help, similar to the masking tape suggestion.

Keep moving the cyl around, or move around the work yourself. Use a draw file technique - grab that barstar* like a pair of handlebars and push away from your body.