As of my last knowledge update in September 2021, the fastest way to gain Crafting experience in Old School RuneScape (OSRS) may have changed due to game updates and new methods. However, I can provide you with some methods that were efficient at that time. Please note that you should check the OSRS subreddit, official forums, or community resources for the most up-to-date information.

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1. Glassblowing: Crafting molten glass into various items like glass orbs, lantern lenses, or empty light orbs is a quick and relatively cheap way to gain Crafting XP. You can do this by using a glassblowing pipe on molten glass, which is made by smelting soda ash and a bucket of sand in a furnace.

2. Spinning Flax: Spinning flax into bowstrings at a spinning wheel is another fast method. It's not the cheapest option, but it's quick and provides decent XP rates. Flax can be picked in the flax field south of Seers' Village.

3. Gem Cutting: Cutting and crafting gems into jewelry can be a profitable and fast way to train Crafting. You can cut gems like sapphires, emeralds, and diamonds into amulets, rings, or necklaces.

4. Battlestaves: Crafting battlestaves by attaching orbs to battlestaffs is a good way to gain Crafting XP. It's not the fastest method, but it can be profitable. You can purchase battlestaffs and orbs, then combine them in the Crafting Guild.

5. Leatherworking: Tanning and crafting leather items like chaps, vambraces, and bodies can provide decent Crafting XP. Collecting dragonhides from dragons can be a profitable and efficient method.

6. Silver Jewelry: Crafting silver jewelry, like silver tiaras or unholy symbols, is an alternative method that provides decent XP rates. You'll need a silver bar and the appropriate mold to create these items.

7. D'hide Bodies: Crafting dragonhide bodies, such as Green, Blue, Red, or Black d'hide bodies, can be both profitable and offer good XP rates. However, this method requires a significant investment in dragonhides.

8. Crafting D'hide Shields: Creating dragonhide shields, such as the Green, Blue, Red, and Black d'hide shields, can also provide good XP rates and may be more profitable than d'hide bodies in some cases.
    

It's important to note that the fastest method may vary depending on your budget and whether you prioritize cost-effectiveness or speed. Additionally, game updates and new content can change the optimal training methods, so always check the OSRS community for the latest information and guides on Crafting training.