To keep the cauldron lightweight and to use less foam, cut up and fill the inside with cut cardboard. Spray insulation foam into the cauldron filling the gaps in between the cardboard.
Continue adding foam to make it look like the witch cauldron is boiling over.
Add Christmas ornaments to the still wet foam and allow to dry. Once the first round of foam has dried completely add a blob of foam only at the top/center. Fill the foam with your remaining ornaments.
Prime your foam and ornaments using chalk paint or primer. This may take 2-3 coats of paint/primer as it sort of separates from the foam as it dries. Once the foam is well primed follow up with a solid coat of green craft acrylic paint.
Embellish the still wet paint with lighter green, yellow and white paints to create highlights on the bubbles. Seal with a heavy coat of glossy polyurethane for a wet look.