Advice please

This is my first cycle taking OPK tests, so forgive me if this is a dumb question. The line is starting to get darker (not positive yet), but how many days before a positive OPK do we need to start BD’ing? Do we start a few days before or wait until the positive? Again sorry if this is a dumb question😂🤦🏼‍♀️ Any advice is appreciated!