The easiest way ro do it is to take a cocktail stick/toothpick and push it through the muzzle brake holes sideways. Then when aligning the muzzle brake just make sure the cocktail stick is horizontal. Saw that in a youtube video
Work process in steps:
1. Assemble barrel and breach (without the muzzle brake)
2. Let the glue dry/harden
3. Assemble as much of the rest of the kit as needed to make sure you have a definite position for the breach and gun barrel.
4. Get a straight stick which is sufficiently thick to get a fit which is not loose in the side openings of the muzzle brake. You don't want it so tight as to have to use force to get it into the holes in the side. Test first, before step 6
5. Set the barrel, breach and whatever else that the breach is attached to on a level surface. Basically a tank or anti tank gun in ready to fire position on a horizontal battlefield. "Gefreiter Schmidt, where ze hell is ze muzzle brake!?".
6. Push the muzzle brake onto the barrel, or screw it on if you have one of the threaded ones.
7. Push the stick through the side openings, from the left or from the right doesn't matter.
8. Adjust the muzzle brake until the stick is horizontal. Check that the mountng (tank or gun carriage) is still horizontal.
9. Apply glue to glue the muzzle brake onto the barrel.
If the end of the barrel that goes into the hole in the breach had been circular then the process is to assemble barrel and muzzle brake first.
5. Set the mounting, tank or anti-tank gun carriage on a horizontal surface. See step 5 above.
6. Push the straight stick through the muzzle brake, left to right or right to left
7. Push the barrel into the circular hole in the breach
8. Adjust until stick is horizontal while making sure that the mounting is also level
The stick is used to allow you to "see" the position of the side holes on the muzzle brake while looking into the barrel from the front or "business" end. If the stick is not horizontal it means that the side openings are not level with each other. The length of the stick also "amplifies" any levelling faults and make them easier to see. If the stick looks level you will not be able to see any levelling faults on the bare muzzle brake.
If you are unsure you can measure the distance between the "battlefield" and the ends of the stick or you can put the ends of the stick on blocks of equal height.
The tricky part will be to get the glue into the gap between the barrel and the muzzle brake ...