Rags or Old Clothing (which you can pick up at most local textile shops or factory for cheap or free, check with Goodwill or Salvation Army, if using old clothing make sure to take any sharp objects off like zipper etc
A Broom Stick or Baseball Bat
First you hang the bag up by one or two of it's attachments and start filling it with cloth, use 2 hand fulls then start packing with broomstick or baseball bat for 20-30seconds and another 2 hands fulls of cloths, keep the process going until you are done. The whole process should take half an hour to an hour or so from start to finish.
The technique to use in packing a bag is going around in a circles over and over, first around the outside edge working your way to the center then do it again, then pack the center. then 2 hand fulls of rags and pack again.
You can add sand bags for more weight, some bags my clients rip open to reuse the bags fill are almost 30-40% sandbags. If you plan on adding sand bags, use heavy gauge industrial garbage bags double lined, fill then with 10-20lbs of sand and wrap the bag with foam underlay so if the sandbag ever settles to the edge of the bag you never have to worry about injuring your hands or feet.