Once you have reached level 45, you can start blackjacking. There are three types of blackjack available in Old School Runescape - Oak, Willow and Maple Blackjacks. Each of OSRS GP them could be transformed in one of three different specializations offensive, defensive, or just regular. It's not what you should be looking at as you train all of the specializations are acceptable, just make sure you're using the maple as this type is the most effective. The Grand Exchange will be the shortest way of obtaining this item but you can take the time to complete the initial part of Rogue Trader minigame, which rewards it.
After you've obtained an edge in blackjack, you'll learn how to make use of it. As mentioned before, blackjacking is the process of knocking your opponent out to steal his money (twice) and then repeating the process to build up the experience. In simple terms, you have to right click your adversary for knocking him out, and then click again to steal his money once he's unconscious.
Since in OSRS the bandits are only dazed for three seconds, you must take them off as fast as you possibly can. If you fail to do so twice in a short time, could result in being arrested. It means 5 second stuns and damages. In the course of time, bandits are also inclined to seek revenge on you, which can result in a message saying to them "im going to kill you to pay for it" and eventually them attacking you. This is averted by taking the right actions.
When bandits says words in the above paragraph, all you need to do is try another knockout. Since the time between sending a message and an attack is extremely short, another knockout has to buy RuneScape gold be done right following the text. If you can manage to get your enemy unconscious it is recommended to use the pickpocket two times as quickly as you can. Repeat this procedure to gain experiences.