It doesn't take anything formal from you. It's best for your family to know your plans and I think the hospital likes to have some record of what might be coming but as long as a body doesn't have certain diseases at the time of death (and some pretty contagious things can be allowed) then it's just a matter of a phone call or two and some papers to be signed. I worked with a family who couldn't afford burial and death was not anticipated and they went with body donation as a practical measure. That's how I actually first read about it even though I'd been interested for a long time.