You roll a rectangular patch flat, wait for some time to cure itself a little (till it is not too stiff and not too sticky) and wrap it along the legs as you want. Then you let it cure fully, remove it (add some scratches and holes if you want it battle-worn) and super-glue it definitively. To prevent GS to stick on surfaces, you can use water with a drop of dishwashing liquid IIRC (or saliva).
This has the advantage over sculpting directly a folded one (or buying a plastic) is that you can engrave or model details on the rectangle that will be realistically folded. And you can drape the tabard the way you want around any leg shape.