You can either sketch (draw the outline in pencil) of each item on paper in the positions you want them or bring all the kit in and let them measure the items exactly. Their office isn't very far from me, so I choose to bring everything in. I lay each item out the way I want them to be presented, take a photos and then e-mail the photos to them, so there are no mistakes. the hardest part really is choosing which case you want and making your mind up on how you want everything fitted.
They do an excellent job customizing everything the way you want it, their turn around time is quick and they enjoy their work. All my HK weapons now have custom cases and each new one I get seems to be better than the last. They're really getting good at their jobs.