My home-brew method if you don't have a vice is to use a table or in my case, kitchen counter. I'd mount the barrel on the upper receiver and tighten the clamp-on FSB enough so you can still make adjustment. Then flip the upper & FSB upside down. Once the ears of the FSB and the sight channel of the upper rest flat on the surface, then you can tighten the hex screws to really tighten the FSB in place. Then you can use your rear sight and FS post to aim at a plum vertical line (e.g. corner of a room where two walls meet) to double check the alignment.