I agree with Pekka. Trot off to your nearest suitable shop, get yourself a good quality 10" or 12" flat second cut file and handle, a good straight edge (solid, as in not flexible, steel rule) a scriber, and a square. At the newsagent get a broad tip marker aka felt tip pen, I like red or black, but anything is fine really.
Do you have an engineering vice? If so good, if not that will be probably the most expensive thing you have to get, a good 3" one will be better than a cheap 4" one.
Select an edge of your piece of plate, using the marker apply some ink to one face. Using the straight edge and the scriber, scribe a line through the ink, as close to the selected (sawn) edge as possible. Put the plate in the vice and start filing, try and keep the edge as straight and square as possible whilst working the whole edge down towards the scribed line. When you get pretty close, so that you can still see a tiny bit of ink, that's the time to start draw filing, as shown in one of the links in Pekkas post. This will, given practice enable you to get the thing dead straight, dead square. Check straightness using the straight edge and a light, hold the plate up to the light, place the straight edge on the filed edge, and try and see if there's any light showing, check squareness using....you guessed the square. With patience, and no little effort you can get this edge to being straight and square fairly quickly, BUT DO NOT HURRY IT!! Especially if you haven't done much or any filing. This first edge becomes the datum edge. Select an end to become your second edge, apply marker ink and use the square with the stock against the datum edge, scribe a line as close as you can get to the sawn edge. I'm guessing that you can work out what to do next!!
To remove the sharp edges a couple of strokes of the file will ensure that no blood is drawn.
Don't panic if it doesn't go brilliantly straight away. Filing is an art, the only way to learn how to do it, is to do it, and be fairly critical in your assessment. I learn't how to file 50 years ago, and I still get pleasure and a nice warm inner glow (as well as the hot outer glow!) from doing exactly what you are trying to do.
best of luck
Bill