The first matter of business is deciding which wall or where to place the screen. This is crucial as it is the focal point of the entire theatre. Make sure the wall is smooth and large enough to incorporate a big image.
With that sorted it would be best to get the projector in place. Whether you wish to hang it off the ceiling or run it from the back end of the room is completely up to you. Set it up and give it a test. It will help you avoid changing the settings later on.
Now is the time to set up the surround sound system. If image quality is the most important thing in a theatre then quality sound is a close second. Set up all the speakers according to the manufacturers recommended settings. Give them a sound test as well and make sure each individual speaker is up and running. There is an option with this on every surround system but it depends on the manufacturer as to where they place it on the options in the menu.
Bring in the furniture. This should include low light lamps, sofas and recliners, nothing more. Set them up so that everyone has a clear view of the screen and space them out slightly to have small tables in between. These tables can be used for placing snacks and drinks and other items. Be sure to do a test run of your home theatre system and make any necessary changes that might be needed.