Attach a cut off wheel (reinforced dremel type is best) to a drill press. Put your guard flat on the drill press table and use the table as a guide. Adjust the height of the cut off wheel on the side of your guard where you want the groove to be. Use the cut off wheel to make a groove all around the guard. It will be straight everytime and you can't really screw up as long as you keep the guard flat on the table. The only thing you have to work on is controling the depth of the groove. Then you can use a file to make it cleaner, wider or rounded if you want or you can just leave it as is.
In this picture I used a small roun file to make it rounded and wider
Here I left it as is and blackened it to give contrast