I strip clean and rebuild all my early WMS drops and the method that works for me is to;

- have the frame completely out of the game
- full strip and clean
- mount the vertical plastic guide / divider strips to the back of the frame
- hold the frame with the tangs for the springs pointing up
- lower the drops into position, hooking each springs as i put them into position - gravity holds them
- with all drops in place, keep holding the frame with tangs up so the drops are suspended by the springs
- put the middle retaining metal strip across the back of the drops and secure with just one screw
- repeat for other retaining strip
- now you can move the whole assembly and drops don't fall out as you install the rest of the retaining screws

Good luck - it's not as hard as it sounds