To unlock a Lord, you have to have the necessary lord pieces.
Example: If you want to unlock Robin Hood, you need to have a previously specified amount of Robin-Hood-pieces.
Your Lords are educated with quality-pieces. These can be used to educate any Lord with the corresponding quality, unlike lord pieces, which can only be used on the Lord they are tied to.
Example: Robin Hood owns a Common Quality, which means, that Common Quality Pieces are needed to educate him.
To educate a Lord, you have to view the Lords details in the Hall of Fame. There you will have an overview of all your pieces.