First you'll need to work out how you want to attach them: that was my biggest problem! Otherwise it's not too hard: for the slot-in type, cut something stiff and weatherproof to the right size and pop them in. External-grade 6mm ply is suitable, or perspex-type stuff. Someone also has a great flip-over type on here: those carpet-over-rubber doormats nailed to the top and bungee-corded on to hooks at the side when it's windy, carpet side to the buns.