Start with how you know they're under-performing. What externally visible indications do you have that they're not working? Eg, if bob was working well at WFH

Start with how you know they're under-performing. What externally visible indications do you have that they're not working? Eg, if bob was working well at WFH he'd do X, Y, Z, but he only did 0.3 of X. You could even compare that to other employees to show that it's just this one person.