How to file an extension?

I'm working on a small businesses books and agreed to file their taxes (it's a close friend). They didn't touch their books(QBO) at all last year and they are incredibly messed up. I need more time. I can't figure out how to file a tax extension and I'm stressed about it (I know ...I waited until the last minute). Does anyone know how to file the extension, the IRS website is a convoluted mess and I can't figure it out.