Option 1: Grab some waxed baking paper and a rolling pin or something hard and flat to squish the green stuff flat.
Put a small sausage of green stuff between two pieces of the waxed paper, and roll or squash it down to an appropriate thickness.
Peel back the first layer of paper, and use a sharp blade dipped in water to cut the flattened green stuff into a long, thin rectangle.
Using the knife blade or tweezers, tease up half of the rectangle strip off the paper and fold it over the other half.
Tease the whole thing up off the paper with the knife or tweezers and push it gently into place on the model's belt, flattening it down a little if appropriate to hang properly.
Option 2: roll out a thin sausage of green stuff. Push it into place on the model, with one end at the belt and the other end hanging downward along the leg.
Use a sculpting tool to flatten it out, and add a crease down the sides with a knife blade or tool.
Press the top inwards against the model's waist to look like it is tucked in over the top of the belt.