First, determine what size(s) you would like your bags to be. Cut your fabric into rectangles that measure the desired width of your bag (plus a bit for seams) x twice the desired length (plus a bit for seams). For example, if you want 5" x 7" bags, cut your fabric into roughly 5 1/2" x 14 1/2" rectangles.
Next, run the short ends of your rectangles through the serger. (You can skip this step if you are using fabric that doesn't ravel.) To save time and thread, serge in serial fashion. Serge the short side of a piece, and when you reach the end of it, put in the next piece without cutting the thread tail. Just keep stitching one after another.
Once you've serged one side of all the pieces, clip the thread tails to separate. Repeat for the other short sides.
Fold your rectangle in half, right sides in. Serge the sides closed. You can do these in serial fashion as well. Turn right side out. Your bag is complete!
Fill your bag with treats. Mmmm... candy.
Lastly, tie treat-filled bag closed with ribbon. I made bags out of several types of fabrics (some you might recognize from my Novelty Cottons post). I am curious to see how the trick-or-treaters react. :)
Post-Halloween update: The trick-or-treaters loved the bags. Even the teenagers used words like "cool" and "awesome." :)