You only want to be able to reach the crafting scene if you have a specific item and activate said item. Am I reading that correct? If yes.
First go to the 'Common Events' tab. Create a common event and name it something like Forge or Oven leave the trigger as none. Add one event command, on the third page 'Script...'. Paste in the following line of code:
$scene = Scene_Craft.new('Forge')
Now go to your item database and go to the Forge or Oven entry and select the common event you just made in the list box.
The string of text inside the parenthesis is just something I thought of you can change it to reflect what you want the text to be at the top of the scene.
Good luck with it cloud read! :afro: