View B Shirt Placket Interfacing

Hello, I'm having a tired daft moment. I've followed the instruction to cut 2 x 1 3/4 x 8 inch interfacing for the placket for the shirt view b, but I can't see when and where to attach it, please could someone advise me! I think I could work it out but I'd rather get it right first time and avoid wasting fabric! Thank you, Helena