Between step 2 and 3 you should map your rooms and rates, check everything and import existing bookings from Booking.com to block the booked dates.
3) You enable the connection in Booking.comBeds24. This is when Booking.com removes your inventory and rates and you are temporarily not bookable.
4) Beds24 sends prices and availability to Booking.com. You are bookable again. To send the update immediately click the "Update" buttons in the channel manager settings.