You can gets started with this video: www.youtube.com/watch?v=45JhacvmXV8. I don’t reccomend waterproofing your stuff though, as it prevents you from modifying them later.

For moulding, coffee ground and glue is simpler, but will take a week to completely dry. It also shrinks, so you have to leave your items inside for the first days of drying. Cardboard layers will dry faster and keep their outer shape, but is more tedious to make. Maybe you can try cardboard inside and coffee ground outside

In any case, cover the surface with paper and the corners with thin strips of paper. I’d recommend drying it quickly (using a heat gun, or the sun) to prevent fungal growth inside the thick parts.