Pedro,
Congrats on being persistent!! Keep it up. There are many ways to make a guard. YouTube has a few for you to watch too. For me, on a narrow tang knife, I cut my guard from a 1/4 inch x 3/4 piece of nickel silver (my favorite material, although brass is easier to work with). Lay out the center line with a ruler and a scribe. Make sure it's center from one end to the other. Use a center punch to mark your holes and use a drill just a little smaller than the thickness of your blade to bore holes. Make sure that the two holes at the end are about the width of your narrow tang. Place the ends of your guard (not the face of your guard) in a vice, and file all the left over material down to the hole edge on both sides. Now you should have a rectangular looking hole that your knife won't fit into (yet). Make sure your narrow tang is a little thinner than your actual blade is up to within about 1/4 inch of the riccaso (broad surface of the blade just in front of where your guard will be). Now comes the tedious part. I don't have a milling machine so I file both sides of my rectangular hole evenly and keep them flat and straight widening the hole and then testing on the tang. Once the guard will begin sliding down the tang (that's thinner than your riccaso), the real patience comes into play. Keep opening the hole along the rectangular sides until the guard slips down to that 1/4 full thickness part of your blade where the finished guard will rest. When you get really close and have to force your guard on to the flats where it will rest, you'll notice guard material left on the blade when you take it off. This indicates where on the guard you need to file more. work slowly. Find a short piece of galvanized pipe smash one end down so that the opening is a little thicker than your blade. Place the pipe over your guard and drive it into place on your knife. If you need to take it off, you can by tapping on each end of the guard with a dead blow hammer working it back and forth toward the but of the tang. I'd leave it in place when I first drive it home and then fit my handle.
Happy making!!