Trimming The Rails
  The receiver flat comes to us with its upper rails too wide. I imagine they do this, because they can't predict what's going to happen when that flat goes into a bending jig whose dimensions and tolerances they have no control over. Regardless we now have the task of fitting them to the bolt carrier so that a) the bolt carrier rides smoothly back and forth and b) the bolt in the bolt-carrier is centered at the ejector and mag-well.
  First thing we need to do is make sure the bolt-carrier can drop into the receiver. To do that we need to make cut-outs that look like these (above) that are exactly the shape needed to get the bolt carrier to drop down onto the rails. I'll reiterate that the cutouts must not be bigger than necessary. If they're too long then when you fire the AK the bolt-carrier will travel to the back of the receiver and pop out. It won't hurt you, but you'll have a single-shot AK which is not our goal here.
  I like to scribe a line to follow on the tops of the rails. To get this line straight, I use my calipers. I don't even turn them on. I just open the jaws wide enought to mark the distance between the outside edge of the receiver and the inside edge of the cutout I made to drop in the bolt-carrier. I then run the calipers along the top of the rail from the cutout all the way to the trunnion dragging the front caliper on the steel. It makes a neat line that I can see easily when I'm cutting.
  Now we very slowly and very carefully ease the dremel tool with cutoff wheel along the line we scribed cutting the rail as we go. I usually go through three cutoff wheels in this step. Be careful to not bind your wweel in the steel, because these Harborfreight rotary tools are only good for three or four builds anyway. No reason to hurry along its demise. Once we get the cuts done we eyeball them and using the flat surface of the cutoff wheel we gently grind down any protrusions we made inadvertantly. Last step is to use the flat surface of the cutoff wheel to buff out any rough edges of the upper rails. Fit the bolt-carrier, and see if it now rides the rails smoothly. Often it will bind at certain spots, and we'll have to do a little fine trimming to get the rail to smoothly slide all the way from front to back.
  A warning here. AK-Builder often sells its flats with ejector rails that are made for the AKM as well as the AK74. This means that for a 7.62x39mm AK the ejector tip is too long and the bolt will hang up when it hits the ejector tip. You have to VERY CAREFULLY file it down to get the bolt to slide past it without binding. If your bolt-carrier binds here be sure you check the ejector tip before you trim the upper rails to get it to stop binding. The problem might be the ejector tip and not the rails after all. You don't want to ruin your upper rails.
  I strongly recommend you wear a filter mask and eye protection during this step, because there's going to be a LOT of hot, sharp bits of metal dust flying into your face. Breathing this dust is almost certainly harmful to your health, and getting a hot steel shard in your eye is REALLY painful for several days.
  This is the way the bolt-carrier should fit. It should not bind anywhere, but the tolerances should be fairly close.
  This point is one of those WOW! moments in a build. The receiver is complete to the point of being able to install the bolt in the bolt-carrier, put on the gas cylinder handguard, install the bolt and recoil guide and spring, then cycle the action. It should work fine at this point.
  And no, that feeling never gets old.
  * This is My Rifle *