...
- Disconnect the chain and unscrew the white nut by turning clockwise. (Reverse thread).
- Remove the handle assembly
- Remove the metal screw where the lever connects to the handle. This is tight! Use impact driver and a vice if you can.
- Remove the white section and add silicon grease where it makes contact with the metal handle.
- Lubricate the metal shaft.
- Reassemble
Optionally, you could replace with new handle for about $15 on Amazon. Probably a better long term solution.
https://www.amazon.ca/Kohler-GP1034693-CP-Cimarron-Chrome-Finish/dp/B001GZVTBC
...