To enter the Crafting Guild, you must have at least level 40 Crafting, and be wearing a brown apron (obtainable from Varrock clothes store, or Malignius Mortifier after Swan Song). If you persevere and prove your workmanship by reaching level 99 Crafting, you can wear your Skillcape of Crafting instead of the apron.
The Crafting Guild is located south of Falador. The guild consists of 2 floors, both with useful items for master crafters.
In order to enter the Crafting Guild you need 40 Crafting as mentioned in the Introduction. You also need to wear a Brown Apron in order to enter the Crafting Guild, but you do not need to wear it after you get inside. It's just needed to go in every time.
There are many things in the Crafting Guild that may be useful to you. There are two floors to the guild.
The First Floor, which contains some Crafting tools along with its own mining area, and the Second Floor with some more items and a Tanner.
On the First Floor there are many Crafting tools that are just laying on the tables around the room. These items are:
Besides these items there is also a sink against the South wall, a pottery oven in the northwest corner, and pottery wheels in the northeast corner. There is also a mining area on the First Floor. To many people, this is the main attraction of the guild. It is shown in the picture below.
It contains three different minerals: gold, silver, and clay. All of which come in handy to people wishing to train their Crafting skill.
There is also the Second Floor, which can be accessed by going up the staircase in the middle of the First Floor. Even though this floor is much smaller than the First Floor, it has its own special features such as these items:
You may also talk to the Tanner up on this floor in order to get any hide you may have tanned, for a price.
Inside the Crafting Guild is the Master Crafter. He is the person that can sell you the famous Crafting Cape.
Just as he says above, once you get 99 Crafting you can come to him to purchase a Crafting Cape and Hood for 99,000 gp.