How exactly do I apply lemon polish to an ebony fingerboard? Also, how often should I apply it?

Don't use lemon polish like Pledge. Use lemon oil or boiled linseed oil. Just wipe it on with a clean rag, wait 5 minutes, and wipe any excess oil with a dry rag.

Remember- When using Boiled Linseed Oil (available at any paint supply store) that the board is clean because it will seal in dirt. Secondly and most importantly, follow the directions on the back of the can for proper rag disposal.

Linseed Oil will spontaneously combust into flames if the wrong method of disposal is used.

So be a pro and follow the directions.