I would assume that you could put these directly on the lawn but seems like that would stain them. Why not put cushions on benches? That would be much more comfortable then sitting directly on the lawn.
Very easy to do. Just buy washable fabric of choice, sew into desired shape and use a zipper on one side to remove the stuffing for washing. Fill with a pillow form, fiberfill or old t-shirts. For a no sew option, use washable fabric glue and Velcro to hold together.
You can pick up some outdoor fabric like Sunbrella and stitch the fabric into the shape you want then stuff them. Or you could try recovering existing cushions with outdoor fabric, like a slipcover.