Here's my step by step:
What you need:
Spaghetti strap tank-tight fitting (without a built in bra). I think the one size camis work great!
First put your tank on its front & cut the straps off at the back where they're attached to the body (you could also probably use a seam ripper & detach but I didn't see the point in taking the extra time for that!)
Turn your tank around front facing you & measure about 1.5-2" up the straps from the body of the cami. Make this into a loop & pin. (DON'T CUT YET!!!)
Carefully try the tank on over your most used nursing bra & see if the loops will fit thru your bra clasp. I did mine about 1.5" long because I prefer it not to slide down so that if I need a little cleavage coverage with a shirt I'm wearing, this could assist.
Adjust as needed then cut strap & sew. (I'm sure you could also sew than cut if you wanted to be extra sure your length was correct.)
Done! Took about 5-10 mins huh? Your next one will take even less time! Now go cuddle that baby & Happy nursing!!