Using a flat stick, cover the tesserae of the joint (here in ecru color for a softer look). Also cover the edges and inside of the "hole" where the candle will be installed (this way, the polystyrene will be "fixed" and won't let little balls escape).
Do not let dry, and remove any excess with a slightly damp sponge.
Finish with a clean, soft cloth to remove any traces of paste from the tesserae without damaging the joints.