I would agree with One of Many, that a scraper is what you need to use to create a flat. The best part of a file is the end, once you sharpen and hone the edge. It should be ground square to the filecut faces, with a slight bit of convexity, so that the corners do not scratch when scraping. I also usually sand the file cut teeth flat for about 1/4" back from the end, so that they surface of the file is smooth leading up to the end edge. When the edge is very sharp, you can easily push off light layers of metal without an extreme effort. At this stage of making a flat, you should be past the 'extreme effort stage' anyways
Unfortunately, a regular file does not hold a good scraping edge for very long, so you'll get lots of practise sharpening the scraping edge.
Hold the scraper at about a 45 degree angle to the surface. Push it ahead to scrape exactly where you want. Lift the scraper on the return stroke, otherwise you just dullen it.
I don't believe that a regular two handed grip on a file will ever produce a flat. You can get better accuracy by applying pressure to the back of the file immediately over the area where you wish to remove material. Use the other hand to pull or push the file, but the stroke should still be very short. You must maintain the fingertip pressure over the area you want to remove the high spots from. For this, you will want to use a thin mill file, one that is flexible enough to bend, so that your fingertip pressure has the desired effect in making the file cut mostly underneath your fingers.