Make sure that you projector, screen and speakers are the focal point of the space. This can be as simple as a white sheet on an old clothesline or taping a large sheet of white butcher paper or foam core to the garage door or fence.
When it comes to seating, you have options. I like to provide director’s chairs, lawn chairs and bean bags for my guests. If you only have a few, ask your friends to bring some.
Instantly make the whole space feel classier with a large rug. Set this down as a sort of guide of where guests should sit. If you do not want to move your big living room rug, use an old bedspread. Not only will this add to the atmosphere but it will keep your guests (and your stuff) off of the grass.
I like to have a basket of quilts for guests to keep cozy and tons of pillows. The big floor pillows are great to sit / lay on and the average sofa pillows are perfect for stacking behind your head.
If you feel like the space is missing something, use a vintage reel, clapboard or a megaphone as accents. If you want something simpler use vintage globe string lights to add a soft glow to your party and have a few sparklers and/or glow sticks laying around. But do not keep the area too lit up because the darker the space the brighter the screen will appear.