Weed barrier is placed on the base, together with gravel if you have it. I also dose mine with slug pellets as birds don't go in the cold frames but slugs do.
In the photo, my original homemade cold frame, with rolling polythene cover is shown on with a newer one made from polytunnel plastic over a timber frame. The timber one had the inside painted white. The plastic-covered design is ideal if you don't yet have a polytunnel. Make sure it doesn't get too hot in sunny weather by adjusting the hinged lid
Also shown if some of the early (for this year) polytunnel plants. Hanging baskets and window boxes or containers can be grown at an accelerated rate in the polytunnel and then acclimatised outside during warm days until all threat of frost has gone (usually end May in lowland SE Ireland where I live).