Litter Training... and other fleece questions

I really want to put fleece in my cavy's cage, But I don't know how to litter train cavys, or or how long after you litter train them do you wait til you put fleece in. Could someone tell me how to?

More questions are - How do you clean the fleece (won't laundry detergent irritate their skin???)
How often do you clean/change the fleece?
Will the cavys gnaw on the fleece and ruin it?