I'm guessin you're the reseller, so from App Studio you'll create the App Studio for them by getting it from the Gallery. Then if you're not planning to go onsite you'll have to get a contact at the customer site and create an account for them in in App Studio by using Invite Customer button. After you've invited them you'll share the app with that account. You'll also need to allocate some licenses to them. Watch out you allocate the correct number of licenses.
Once that is all done the customer can log into app studio (Chrome is probably the best browser to use btw) and they will see the app you've created for them. They should add their devices on the devices tab. Then from the App tab they can select the install button to install it on those machines.
If they use a proxy on their MFDs they'll need to set that.