Hmmmm.... I think that I watched it but couldn't find it. someone did explain it to me. but I haven't done this on my own yet.
Start with a Surly Junk Strap inside your bag. Let's say you're starting with the right grommet.
Thread the strap through the grommet to the outside the bag, around the horizontal rack rod (I'm sorry, I don't know what it's called exactly. It's the rod that you would "clip" a pannier to) and back into the bag.
Thread the strap through left grommet and out the grommet, around the horizontal rack rod and back into the back into the bag.
Secure the strap into the buckle inside the bag.
Roberta