Here is my wood. I painted the edges because I wasn't planning to cover them with fabric.
I used mod podge and covered the entire board with a layer first, then I smoothed my piece of fabric which I had cut larger than the wood over the surface. I haven't yet figured out a way to work with mod podge without having a thin layer all over my hands-so don't be afraid to get messy. I made sure to spread the mod podge past the edge of the wood onto my fabric because I knew when it dried it would make the fabric stiff and this would be helpful when it came to trimming down my fabric.
While everything was drying I cut a rind shaped piece out of foam core board as well as my seed pieces. I just eye balled these. You could make a rough pattern first if you wanted. I painted those the appropriate colors and let them dry as well.
When the watermelon piece was dry I turned it right side down on my cutting matt and trimmed the fabric close to the edge of the wood all the way around the watermelon with an Exacto knife. Then I used hot glue to attach the rind and the seed pieces. Tada!!!