Somehow several years ago I become the family gift wrapper, and have had quite a bit of experience since then. So here are a few things I've learned. I hope they help!
Also, I've got a few different decorating ideas in the last step.
Step 1: Measure and Cut
Now, flip the gift upside down on the wrong side of the wrapping paper, so that the top is facing the plain side.
Step 2: The First Folds
Step 3: The Bottom Edge
Fold one side in. and crimp the two edges that you've made. Do this again on the other side. (I'm not sure if that makes much sense, but hopefully the pictures help here.) If you were looking from the top of your package, it should look like \_/ or, if your package is smaller, it might be just a v.
Fold the top flap down. On the bottom flap, fold the raw edge over (Like you did in the step before this). Then fold it up and secure with tape.