When you check-in, you will be asked to weigh your suitcase or bag, and asked to attach a label to it that will have a specific ID and flight information. You are often given a sticker which is usually attached to your passport with details of your baggage so that if it gets lost, you will have records of its details. The check-in staff are on hand to assist.
If you have already checked-in online for your flight, you will be able to do the same as above. However, you will need to head to the bag drop off point instead of the check-in desk, unless otherwise instructed. At the baggage drop off point, you will be instructed to attach a luggage ID label and place it on the conveyor belt where it will be weighed and taken away to be processed.