If you have pets who tend to rip up their beds, providing them with a nice cheap bed that they can rip up all they like is the way to go; you can just make another one whenever the current one gets ripped up!

[edit] Steps

Find two used cloth laundry bags. These will be enough to make one pet bed.
Stuff one laundry bag with four old pillows (layer two over the other two). Pull the string and knot it to keep it closed.
Place the knotted bag into the other laundry bag, with the tied up end going in first.
Try to keep the other laundry bag closed, but it's not crucial. When it gets dirty, just slip the outer bag off and wash it.
If you choose to leave the string on, keep in mind that it's a real temptation hanging there for them to chew on. This doesn't help longevity and it could also present a strangulation hazard to your pet.