These thermoplastic pebbles melt at 60 °C - you can use a heat gun or hot water for it. In their mouldable state, they become transparent and can be processed into a mass that holds its shape and regains its white colour when cooled.
After achieving the desired shape, you can let it cool in open air, or put it in ice water to speed up the hardening.
The mass is re-mouldable, just heat it up again and you can even add and work in more pebbles.
Caution: Take care when working with the heated thermoplastic, it can easily cause burns.