Congratulations on choosing a great brand. There is an art to filling bags. I have stuffed a few heavy bags (muay thai, regular boxing, and wrecking ball) Here is a step by step system on the basics of filling a thai bag.
1. Make that trip to Goodwill or other thrift store and ask if you can purchase a large amount of their unsellable shit in the back. After you get your old rags, shirts, towels, whatevers, the fun begins.
2. You will need to sit down with a couple beers and a sharp pair of scissors for this. Cut all the material into small pieces. This is the most tedious and time consuming part of the process. Grab a training partner to help.
3. Time to fill the bag. If there is a chain hooked onto the four attachment rings at the top of the bag, remove it. Now to make the next step much easier: Starting from the top you should roll the leather down over itself, turning it inside out as you double it over. Roll it down as far as you can until the very bottom of the inside of the bag is reachable. This makes stuffing the bag easier because now you can get to the bottom with your hand, as opposed to climbing inside the bag to try to pack the stuffing down with your foot.
4. Stand the bag up and begin to stuff. As you pack the material in, you want to press it from the center, outward and force it down along the inside of the shell. Pack as dense as you can before throwing in the next pile. You will get a tight pack without the