If you're going to use a disposable plastic piping bag, you'll need to clip the tip off before you start. However, do not simply snip it off without first estimating the size of the tip you wish to use; otherwise, the tip may fall through the hole.
Finally, with the tip in the opening, place the outer half coupler over the tip's outside and tighten the screws.
When it's time to fill the bag, most people find that holding the bag across the middle and folding the opening over their hand to make a cuff works best. That way, when you spoon the frosting, mashed potatoes, or whatever into the bag, the ends won't flop around, resulting in spoonfuls of contents spilling down the bag's outside.
The actual piping takes some skill and getting used to, but the general method is to twist the loose part of the bag so that your hand is directly behind the bulk of the filling. Then press lightly from behind. If you squeeze from the centre, the filling will spill out the back of the bag. As a result, it's critical to keep it securely twisted while managing the flow.