Leaving my tenure-track position for industry in January 2021,
When I left I first informally talked to my direct line manager to whom I usually reported (which was in my case the head of the department). Then my official (and signed) letter went to human resources to formalize the termination of my contract. Then talk to your PhD students / postdocs etc .. It was a slightly different situation as I left after I already had tenure but I guess the same procedure might work best for you as well.