If they eat once a week, they generally poo once a week a few days later. As they get older, they eat less often and therefore poo less often. As to "how"... same way as everything else does!
Lestette has it. As soon as you see/smell poop, take it out and any soiled aspen around it, and replace with the same amount of fresh. Then once every 4-6 weeks, empty all of the aspen out, clean the tank with reptile-safe disinfectant and replace with clean fresh aspen.