You should call the school and ask them because each school requires different components and there is a school code that you will need to have and whatnot. I applied to Charity and LSU and for Charity they pretty much did it for and you paid them and for LSU they had a PDF document with instructions to follow and how to sign up. But essentially you will need to sign up at the Elsevier website which is the company that writes the HESI. The website is: https://evolve.elsevier.com/productPages/s_872.html
, but considering some schools require different parts of the HESI like Critical Thinking, or Chemistry, or Vocabulary, and some don't require as many you really should call Admissions and ask because the test isn't cheap and you don't want to have to retake it because you didn't take the right components. But the place I took HESI at for LSU they have openings almost every day.