Measure along the square tubing with the tape measure and place a mark at the center of the miter cut with the awl.
Adjust the blade of the combination square to span across the face of the square tubing, place the base of the combination square against the side of the tubing and scribe a line at the length mark with the awl.
Set the angle finder to 1/2 the degree of the final miter. For example, if you need a 30-degree finished miter, set the angle finder to 15 degrees.
Push the base of the angle finder against the side of the square tubing with the blade of the angle finder aligned with the intersection point of the length line and the side of the square tubing and scribe a line along the angle finder blade with the awl.
Flip the angle finder so the blade forms a V with the scribe line you made on the square tubing and scribe a second line across the face of the square tubing.
Extend the ends of the V across the face of the square tubing 90 degrees from the face containing the V with the combination square and awl.
Connect the extended ends of the V to the scribed center line 90 degrees from the face you scribed the lines in the previous step.
Attach a .040, 4 1/2-inch abrasive grinding wheel to the 4 1/2-inch angle grinder.
Cut along each of the scribed lines marked in steps 5, 6 and 7 with the abrasive grinding wheel to cut the V-notch miter from the square tubing.
Score the inside face of the square tubing that remains intact with the abrasive grinding wheel. The score line needs to have a depth equal to 50 percent of the square tubing wall thickness. Allow the miter notched square tubing to cool before handling and forming the final angle.