It depends on the bus company and pick-up location.
Your order summary will clearly state where you need to go for pick-up. Once you arrive at the stated location, if there is a counter there with a unit number corresponding to that on your order summary, you will need to go to that counter to exchange your order summary for a boarding pass or a bus plate & platform number. Show this boarding pass to the bus driver as you board the bus.
If your pick-up location does not have a counter, and is, for example, a taxi stand, then you may show your order summary (electronic OR printed paper version) to the bus driver as you board the bus.