How tidy guinea pigs are, depends also very much on the conditions their ancestors have been growing up/living in and to a certain extent also on the personality. Some piggies can be more fussy, some can be very scatty.
Piggies have to run their hay/grass fibre based (and NOT veg based) diet through the gut twice in order to digest the fibre fully. They will eat the poos that contain the digestible fibre for the second run straight from the anus (caecotrophs or 'eaten poos') while there are plenty of waste poos from both runs. These are spread everywhere as piggies poop where they go and sleep. You need to poo patrol daily, unfortunately.
More crucial is that most piggies have peeing corners, usually in a corner and where they sleep as they cannot control their bodily functions during a nap. Putting a tray with soft hay or puppy pads in the toilet corners and sleeping areas can help with absorbency. Pegging an old hankie or similar over the corner they have chosen, will make it more enticing to use for peeing.
Another good trick is a shallow tray with soft hay for toiletting and sleeping under a hay rack; eating stimulates the gut. Piggies are often very fond of an arrangement like that. You will have to clean the tray daily though and may want to have an absorbent underlay of some sort underneath the hay to prevent it from becoming a soggy mess quickly.
Unfortunately, you cannot toilet train piggies (they would be basically have to constantly sit on the toilet). And unfortunately it is very much a myth that piggies are 'easy pets'. they actually create more work than other pets if you want to keep them in the good conditions that they should be kept.
