You could use an old dresser. Here's a link:
The sink itself? If you’re referring to a project you saw, please give us more details. Thanks.
The sink itself must be impervious to water & a lot of chemicals - like hair dye & nail polish.
Concrete sinks are very hard to make this way for beginners.
Buy a used sink on Craig’s list or Marketplace for cheap. Spend the money on a decent faucet set.
I think your easiest way to make a sink is to use a vessel that is the right size, shape and character for what you want. Think things such as giant serving bowls, spheres cut in half, already formed concrete like a flower pot, etc. I think the easiest place to start is to have an idea of the aesthetic you're trying to achieve.
I suggest that there not be any holes in the vessel and that the material is thick enough to withstand things as Lifestyles mentioned. You must also be able to drill an opening for the drain to be fit in.
Here are instructions that you can use to guide you with whatever you choose.
Here are images of different upcycling projects creating bathroom sinks. You can sort them by materials at the top.
Do you want to make a vanity to hold a sink or to support a vessel style sink, or do you want to make the basin as well?