In general, most modern vibrato units have been designed for easy string replacement. The older-style Bigsbys (which can be found on many older guitars, as well as new Gretsch hollowbodies) can be frustrating because the string must be attached to a retainer pin, then looped around the string retaining roller bar. The easiest way to deal with this is by pre-bending the ends of the new strings. The quickest “low tech” way to do this is to use a pen or pencil, since those are very close in size to the roller bar. Simply place the ball end of the string on the pen or pencil, holding it in place with your thumb, then wrap the end of the string about one full turn and hold it there a few seconds. Once you let go, the ball end should have a sort of hook-shaped bend. Now you can slip that under the roller bar and onto the retainer pin. The final step is to pull the string tight over the top of the bar and then threading the plain end into the tuner post on the headstock. It’s a little tricky at first, but once you’ve done it a few times, it becomes quite easy.