Filling will be the same for all bags except the parchment triangles. Put the bag in your non-dominant hand, (left if you're right-handed, right if you're a lefty), and use your other hand to fold the top of the bag into a cuff over the hand holding the bag.
Use your free hand to scoop the frosting into the bag. You can use a spoon or a plastic spatula for this, or my favorite tool is the metal spatula. I seem to make fewer messes using it!.
For parchment triangles, you really won't be able to make a cuff like we did with the others. Just fill it carefully using a metal spatula. The most important thing to remember when filling parchment bags is to be very conservative on the amount of frosting you put in there. Parchment cones are notorious for oozing all over the place if they are overfilled.
Disposable and Reusable:
The best way I have found to close these types of bags is with rubber bands. You can use any rubber bands, but the small rubber bands that are included in the Cookie Kits are the perfect size for sealing up bags. They make a tight seal without having to wind the band around the bag a bunch of times. After you've filled your bag, gather up the top and fold over. Take a rubber band and secure it tightly over the folded section of the bag.
There really isn't a super-secure way to close parchment bags, but here are a couple of methods that work fairly well.
Twist the top closed.
Fold the top down, fold the two sides in, and finally fold the top down one more time.