Supplies: I used a large box and cut out the two largest panels with an exacto knife. Your flannel board can be any size you can want, provided you have that much cardboard/posterboard/plywood/etc. I got some "Snuggle Flannel" from JoAnn's on the cheap (it's in the children's fabrics section). I chose this blue pattern because it made me immediately want to put little felt clouds on it. I also bought a lot of felt (recycled felt is awesome!). I used a regular old glue gun to stick things to other things (and sometimes to my fingers).
Process: I measured out enough flannel to cover the box panel with extra to wrap over the sides. For my box, it turned out to be about a yard, cut in half on the fold of the fabric. I cut out the corners and hot glued the flannel on the backside of the cardboard. I didn't use any glue on the front, only on the pieces that wrapped around the back. Then I went about snipping and cutting flowers and some flower pots (which I drew defining lines on with sharpie) and hot-gluing flowers together. Voila! All in all, it probably cost me less than $5 for both boards. (And, yes, that is my bed, covered in embroidery floss. I need a craft room.)