Here's the way I make sights for AoS.
Step 1: Open your gun in slab6
Step 2: Double the dimensions, and start working out the sight, making it look like what you want it to in the png.
Step 3: Double the dimensions again, work out final details, and Print Screen it.
Step 4: Open up the picture in Paint, cut out all of the stuff except for the gimp window (It should just be the sight and a gray-ish background on the picture. Then resize it to 800x600. Use your cursor, go to 400 300, and make sure it's in the center of your scope/where you want the tip of your iron-sight. If it isn't, go back to Step 3, and center it better
Step 5: Save it, and open Gimp.
Step 6: Open the picture in Gimp, add an alpha channel, and use Fuzzy Select to erase all of the gray background.
Step 7: Use the pencil, open up the Tool Options thing, change it to the Pixel shape, and resize it to 1 or 2.
Step 8: Start making the actual sight, and make it look somewhat fitting.
Step 9: Save it as the semi/smg/shotgun .png in the AoS files.
Step 10: Test it out in-game, and make adjustments until you like it.
Step 11: Your done.
Example of a finished sight:
