Practice. There are no shortcuts. Study classic engraving, especially artists like Dore and Durer, and try to emulate what they were doing. Work on your line technique, and using variable widths, and use crosshatching to create value. Practice by filling simple shapes with lines to create values that are close to grey values, like try to figure out how to do 10%, 20%, 30%, etc.
There are no "rules' to it, work out works best for you. Practice using lines and their angles to convey volume, shape and depth. It's pretty challenging, so be prepared to put many years into mastering it - the greats like Dore and Durer did.