One of the things I found handy in a building project, theater or otherwise, is to decide where I'm heading. I think it's useful to figure out what your goals are, make a wish list, and a need list, and reconcile the two.
How will the area be used? Will this be a theater area for small groups, one or two people, or will you be using it for a larger bunch of people with the lights on? Will you be using it to watch classic movies, or for a lot of TV, games or sports? All those things can make a difference. An area that you are going to spend time with the largest interest in watching theatrical movies might require a widescreen with a 2.4 to 1 ratio. Something like 70 to 80% of the hundred best movies since 1960 on a number of lists have been in that ratio. If your interest is in watching sports or gaming, you might want to screen that mimics what current flat screens use with a 1.78 to 1 ratio. Either one will make your main image fill the screen without black bars. If you have a small group, you might lay the seats out differently than a large group, that may take advantage of that cement riser you mentioned, allowing seating on two levels. The screen you use for a group with the lights on may tend to use a slightly darker color to absorb light that hits it, and make the blacks darker. A dark theater that remains that way, most the time will get along perfectly well with a white screen. Some people may argue this latter point, however.
What are you looking for in terms of acoustics? Is soundproofing an issue? What sort of ideas do you have about aesthetics in the area?
I found planning things out made things go smoother during construction.
One other point. Getting things useable tends to slow the last part of the construction down. You'd be amazed at how good a projected image looks on a grey cement wall, and how confortable a lawn chair can be. How you approach this aspect of your timing is a personal decision.