What is the most logical next event?

How to potty train a guinea pig
Observe your guinea pig.
Before you put a litter box in your guinea pig's cage, you need to watch her to see where she normally goes to the bathroom. Guinea pigs like to scent mark their territory, so your guinea pig will likely scope out a corner of her cage to pee and poop in regularly.
Although there is one corner where she will go most often, this does not mean she will always go in this corner. Just pick the place she frequents the most.