Determine if your film has one set of holes or two. If it has two sets of holes (one running down each side of the film strip) then it can be put into the projector in either orientation. If it has holes on only one side of the strip, then it must be put into the projector with that side towards the projector body (away from the operator).
Extend the arms that hold the spindles which carry the film reels (if yours is a projector where they fold inwards for storage). Place the reel with the film you wish to view on the front spindle of the projector with the loose end of the film extending from the front of the reel rather than the back, similar to the way most people put a roll of toilet paper on the holder.
Place the empty, or "take-up" reel, on the rear spindle of the projector.
Take the free end of the film and put it through any idler wheels that are between it and the lens of the projector. There should be a small button somewhere near the lens assembly that allows the film to be pushed through the machine to be engaged by the drive sprocket. Press it down and feed the film through the slot just above the lens (in some machines the lens assembly is on hinges and swings aside to allow access to this slot). If you can push the film through without too much trouble, then you have correctly identified the button you needed to press. It should come out of another slot below the lens, where you should feed it through whatever idler wheels are below the lens.
Pull the loose end of the film through the machine, around the wheel at the back of the projector, and up to the take-up reel. Here it needs to be fixed in place so that the film does not simply pile up behind the projector as it is pushed through. Most machines have a small slot in the take-up reel into which the end of the film can be held while others require you to tape the end to the spindle at the center of the reel. Once the film is fixed, turn the take-up reel a few times until most of the slack is taken up (do not turn it until the film is taut).