Litter training is quite hard, but it can be done nevertheless. Litter boxes can be found at pet stores, they also sell them at most exotic vets (at least the ones I have been too). Basically you fill it with the litter material (aspen is a good choice, although there are plenty of others), and some hay (or other material) that has already been soiled, plus a few poops, what this does is encourages them to go in the litter box. Guinea pigs usually go anywhere and when ever they want, thus why it is so hard. But after saying that, they do tend to go in 1 particular corner. Find this 1 corner and place the litter box there.
I did not intend to litter box my girls, but every day during floor time I placed the cuddle cup out, and at the end of the floor time session, there was not 1 poop or 1 wee in the floor time room, all of it was in the cuddle cup. Intrigued, I put the cuddle cup in their cage and every since they use it as a litter box. It does means that when I am vacuuming the fleece out I also vacuum the cuddle cup and when I am washing the fleece and underlay (in my case mattress pads) I also wash the cuddle cup with 1 cup of vinegar.