First off, make sure you prepare this in advance as projectors need a little time to warm up.
Locate the screen in a suitable place, and consider the light and how that may change as time progresses. Will the sun start to shine brightly through a certain window, for example.
If your projector is portable, place it where you think you need it to be and plug in the laptop (or other device)
Turn on the power. Give the projector time to warm up.
Select the import source, for example Computer (15 pin VGA), Video (Yellow RCA), S-Video
Most projectors have an Auto Adjust button. Press this. Then check the keystone correction (which make the image square).
Now you will need to either move the projector or adjust the zoom.
Focus the projector using the manual slider to get the clearest image possible.
If you need outdoor projection, consider using an outdoor projector housing, which will protect the projector from extreme temperatures, rain, snow, hail etc.