The position of the new bass bar is carefully marked on the plate, and temporary guides glued in place. The contact surface is rubbed with chalk.

The bass bar blank is rubbed onto the chalk.

The chalk is transferred at the high points

The chalk is scraped off with scrapers and planes. I like to use a glass scraper . It is very sharp and i can see the wood through the scraper!

The process is repeated until the whole surface is covered with chalk, indicating a close fit.