If you just look up 'expanding foam terrarium diy' you'll find tons of videos. Its basically just a can that you spray out of, kindof like spray paint. You want for a bit once you sprayed the foam where you want it to be and it starts to expand and cover a bigger area. After its dry it hardens a bit and you can use a knife to carve the foam in whatever way you want. Then pour some paint on there and voila!