Loop the newly-cut strap through the ring on the other one and sew shut. You're done! That was easy, wasn't it?
Step 3: Measure twice, cut once
Now try on the bra and see how long you want the strap to be. Take the ring from the bottom of one strap and bring it over to the other strap. Make sure that you'll have room to tighten and loosen it as you see fit before you cut off most of the other strap.