I'm not sure how many people know this tip but I remember learning and thinking "oh my gosh...genious!" So if you ever need to pipe frosting onto a cake or into a cookie, here is my tip on how to make your life easier and to keep your hands and kitchen clean.
First take a large mouthed cup. Place your piping bag (or you can use a ziplock bag like me) into the cup and fold it around the edges. You want to put it so the corner of the bag is pointing down in the cup. If you are using a certain piping tip, that will face down into the cup. Keep it close to the top of the cup.
Take a rubber spatula and get some frosting. Scrape it into the bag. If you want to add more frosting, keep pushing the bag further down into the cup with your rubber spatula.
Once you have enough frosting in your piping bag, pull the sides up over the frosting and twist to close off.
If you aren't using a tip on your piping bag, just cut off the corner of the bag to pipe the frosting.
That's it! Clean and easy way to fill your piping bag!