Find the right spot

First you'll need to scope out a good place to set up and also one that is nice to sit and won't be bothered by the outdoor elements.  Grass is a nice place to sit, but keep in mind that people like me are allergic to some grasses.  Also, clover grass doesn't do well with recovering immediately after being stepped on.  You'll want to avoid areas where your experience would get disturbed, for example people driving by and a light shines onto the screen, or next to the Air Conditioner condenser where it could get loud.

Projector

I've been to a few outdoor screenings with friends, and I have to say that you really need to get something that has enough power to throw light at the screen.  When it's still twilight outside, the light in the sky will still interfere with trying to see the image.  Some of the cheap projectors on Amazon will sell for less than $100, but those projectors were meant to be seen in complete darkness, indoors with all the lights off, and full blackout curtains. The best way to know if the projector is capable of being seen outdoors is to find a projector that would be more than 3000 lumens (2000 ANSI lumens).  You might also want to consider a projector with a good speaker integrated into it.

Screen

The screen is just as important.  You can just project it onto a wall and call it a day, but if you want to have the image pop out and really see it, then you'll want to get a screen.  The coating on screens have properties that let you see images better because they are able to reflect the light at a better than just putting it up on a wall. 

Screen Stand

Most products out on Amazon come alone, and they can be mounted to walls or hung by rope.  But if you are able to get the screens with the stand, don't worry anymore and just call it a day.  The stands should have weights on them so that the screen setup doesn't topple over on someone if there's a slight breeze.  The stands should also keep the screen stable so it doesn't waver in the wind.

Projector stand

Not a requirement, but it helps that people sitting in the front don't get in the way of the image.  It's really helpful if you want to put your projector in the back of everyone and get a large image to watch the projection.

Blankets

You'll need quite a few blankets for this.  You can use a tarp or a few unused trash bags to cover the grass so they don't get the blankets too dirty.  Place a thick blanket so you have some padding underneath you.  The ground can absorb or give off heat, so we'll need to create some insulation from you and the floor, just like camping.  Then place a large blanket for the floor to cover your ground cover.  Then lastly, grab some blankets to cover yourself for the show.

Pillows or Seat cushions

This one's optional if you have all the extra padding and blankets, but I'm not as young anymore, so sitting on the floor for an extended amount of time is starting to get painful, especially for the back.  Bring something so it's nicer to sit on, like a pillow or a seat cushion.  Or you can keep it to hug or hide during the movie.

Mosquito Repellent (Optional)

If it's summer, usually there's lots of mosquitos outside.  Thermacell has a great product to keep the mosquitos out of biting range.

Recap

Now that you have all your stuff together, enjoy!  It should take about 30 mins to set it all up, but you'll be able to enjoy right away.