What I would do is ask them permission politely to put in on their shirt. I would hand them the microphone and politely ask to insert the microphone in themselves then I would take over in clipping it on their sleeves or placing some plaster tape above their chest. if its a girl i would prefer another girl would do that but if I have no choice then I would carefully instruct the talent herself or himself to do it, i would demonstrate it and let them do it themselves. The key thing is always be polite in asking and people would always agree. Besides when the talent have already agreed to the interview then there is a mutual understanding that it must be done at all cost, but of course, with respect to each other.