I did this about 2 months ago to align my vertical crosshair. I took some fishing line (pretty heavy pound test for larger diameter) and used a sharpie pen to color it black. Using the colored line i hung a small weight (ziplock bag of empty brass shhhhhh dont tell anyone) from a nail on a white wall. The weight garantees the line will be vertical. Set the scope to the lowest setting and look at the line. When you get the vertical hair on the line your good to go. Tighten your rings down, check it again. If no change, bore sight and head for the range. Hope this helps.
Really any kind of line would work. it just has to be thick enough to show up in the scope. I tried using the corner of a wall but I couldnt get one far enough away.
I have way too much time on my hands for figure this stuff out. LOL

I am going to be doing this tomorrow evening when my new rings and rail get here.