Hello Bobsmith
If it's like the one in our Swift Challenger, lift the lid and the front of the 'frame' will pull away from the lip that holds it in place, causing the bottom of the container to drop forward. Open up a new waste bag and place inside the container, then fold the top of the waste bag around and over the 'frame'. Close the bottom of the container up to the 'frame' again and it will clip back into position, close the lid and it's ready for use. When the waste bag is full, just open the lid, lift the front of the frame away from the bottom of the container, as before and put another new waste bag in - simples!
A lot of sites now have recycling facilities, so remember to always have a separate bag somewhere for the cans, bottles, paper etc.