I used the same technic as my Chaos Knights standard bearer. I started by the creation of the Chaos Icon. After that, I drew the banner on paper and painted it with the icon. I stuck both together then I added some details in Green Stuff to enrich the banner. A little bit long to do but the result suits me well.
My DoomBull and his warband are from now on protected by the Chaos Star during their long and dangerous adventures on the road of the power.