I don't know if I do it the right way, but I stretch them. I put them on properly, untwisted, wound properly on the tuners, all that deal. Upon tuning to pitch, I then lay the bass across my lap on its back and stretch each string upwards, wiggling back and forth as I do, about enough force to JUST start to pick up the weight of the bass. I do it down around the bridge pickup, up around the end of the fingerboard, and at three or four more points along the scale. I'll usually do that four or five times per string, and I find that my basses usually stay in tune pretty damn well after a string change, so I guess it works? I dunno, I just always have. Made sense to me.