You log in to the bungie.net website using the Battle.net credentials that you wish to transfer. Once logged in go to account settings then accounts and linking and select the add button next to Steam. Enter your Steam credentials to link your Steam account to your Battle.net account. Keep in mind, if you already migrated one Battle.net to a Steam account you will NOT be able to migrate that Battle.net account to another Steam account. PC Migration was intended to be a one-time function so there is no re-do or cancellation option.