Depends on what kind of ledgegrind you're talking about. Frontal ledgegrind, back-facing ledgegrind, a slim ledgegrind?
Guess I'll explain em all quickly.
Frontal ledgegrind: hold W + lean frwrd
Back-facing ledgegrind: hold S (optional, personally I discourage it), slightly tap lean bckwrd to keep balance while not hitting anything
Slim ledgegrind: slight taps frwrd, kinda like a normal grind.