By a lock and recycle plastic bags. Common size from most stores over the seat, something a bit larger for the bars/controls.
I prefer the plastic hair shower caps for the seat. You can find them in the women's beauty aisle, they come in a pack of 20 or so for $1. They have an elastic band that keeps them in place, and they crumple down enough that you can shove them up in the seat rails for storage when riding the bike. Actually have one stored under all my bike seats.