How do I use a strobe tuner to tune overtones?

I'm trying to build a marimba and the information I've found tells me to triple tune the bars by tuning various overtones. I know which ones to do and how to raise and lower the pitch of the overtones but I have no idea how to measure them to see if they're right. I've been told to use a strobe tuner so I got the app for my phone. I just can't figure out how to use it to measure overtones.