Its easier than you think, normally using the easiest base material being green stuff if you mix different volumes it will dry quicker on slower, if you make it fast drying its more resilient to pressure, I score the surface, mount the GS into the area and then I cut to shape. When its a large flat area like a banner its easy to do.
I will say however i do it in parts so each part of the banner design is done separate and it gets built up into the whole.
@ Duke Danse Macabre Thanks for clarifications I should to try it. I found that the making banner from transparent plastic is more easier way for me. All you need to do is to cut the flag then heat it in the places where you want it to bend and leave it to cool.