You can use velcro or you can weight it down. Most people use clean rocks (nothing sharp or jagged). Just make sure you give them a wash in a little bleach (10% solution should work) to kill any bacteria from outside.
When they are younger they poop alot more so you normally have to switch liners every couple of days, but Maisy is 3 now and is nowhere near as active as she used to be and the liners only need cleaning once a week. Also, I spot clean every morning (just pick the poop and any other mess up). Personally I don't wash her liners with any detergent unless they absolutely need it because Maisy hates it, she will refuse to sleep in her fleece blankets if I used detergent.
I prefer to get lots of fleece so I can accumulate a load and always have them on hand. I have a small plastic trash can that I keep them in until wash day. I wash in hot water with very little detergent. If you buy too much fleece, cut smaller handling blankets out of the leftover or if its very little leftover then cut into strips for their burrowing pleasure. Mine love strips in their kleenex-box houses.