I am making stew for dinner tonight in a crock pot, i want to thicken it with corn starch. how much do i use..?

like i said i am making stew with meat and potatoes, carrots, beans, portebellas, onions, and 2 boxes of beef broth. i want to know how to thicken it with corn starch. how much do i use and when should i add it? please only answer if you know, thanks in advance.