A vintage-style tremolo, like the one on a Stratocaster, is balanced between string pull at the front and springs in the back. Take every string off at once and the springs pull the bridge back against the body. Restring all at once and the balance has to be found again from scratch.
Steps:
- Detune and remove one string. Push it out of the block from the bridge end, then pull it free through the back.
- Feed the new string through the matching hole in the back of the body and pull it through until the ball end seats fully in the block.
- Guide the string over its saddle, under any string tree, and to its tuner.
- Wind with a few neat downward wraps and tune close to pitch.
- Repeat for each string, then tune all six, since each change shifts the others slightly.
- Settle the strings and retune several times.
If you change gauge, the bridge angle will change. A heavier set pulls the bridge forward and a lighter set lets the springs pull it back. Adjust the spring claw screws in the back cavity to bring it back to where you want it, then check intonation. A drop of graphite in the nut slots and under string trees helps the tremolo return to pitch.
Good choices for this bridge include S942 and S1046 in Power Wound Nickel or PS942 and PS1046 in Power Steel Stainless.