You can gets started with this video: https://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. Corrugated 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.

I don't have it with me right now, but basically I just wrapped many layers of paper around the inner part until it tightly fits the cover

That's a good idea, but I'll wait for a few months of use to see if I want to modify it before painting.

I just mixed it with glue to make a grainy paste. Then I used it to fill around the the razor and brush. Finally I covered it with paper and dry for a week. Occasionally I would leave the razor and brush inside to prevent it from shrinking

Electric toothbrush head.

Ingenious compact choice.

That's a Gillette Superspeed, or a modern reproduction. They're about 3 inches long.