Here is the list of quests that give old bags:
http://www.runesdatabase.com/item/209231/old-bag
It matters not which you do - they all give the same reward. Some are easy, some are hard and all of them have 4-7 varieties, so if you run into hard variety, just move to the next questgiver.
Once you have the bags, open them when right combination of classes. Bag opening is smart enough to check what you need and what you have already. That means open them one at a time, since you want to have L15 already learned when you are trying to get L20, you want L20 learned to get L25 and so on. Then use the trainer in Heffner. First two certs at 15 and 20 are universal, but 25+ are only accepted by Heffner trainer if obtained from old bags.
50 is special - that cert will give you two skills, on both sides. In other words, you need to get elites for 15 Wd/s, 20 Wd/s, 25 Wd/s, 30 Wd/s, 35 Wd/s, 40 Wd/s and 45 Wd/s, and also corresponding 7 elites for s/Wd learned. Then your old bag will give you the L50 cert, and using it will give you both 50 Wd/s and 50 s/Wd.
Makes sense?
