Here’s a quick collection of videos I’ve seen recently that offer some form of undershirt advice.
When Should You Wear An Undershirt?
By Ryan Magin (link)
Do You Have To Wear An Undershirt?
By He Spoke Style (link)
How To Rock An Undershirt
Three Types of Undershirts
By Apparel Illustrated (link)
Also How To Style T-shirts and Undershirts (link)