Don't feel bad, it took me a while too, haha
Here's what you do:
Unequip whatever item you're trying to repair, and make sure you have the necessary repair item. (Example: You need a maple branch to repair a Harpoon)
If you don't know what item you need, you can select the item in your inventory, and it will tell you at the bottom of the stat page.
Equip the necessary tool to become the crafting class that creates the item. In the example, a Carpenter would be needed to repair a Harpoon, since they are the ones that create the Harpoons. (I don't know enough to know rank limits, but the higher the better, obviously)
Select the item from your inventory, rather than the Gear page. An option to repair it should appear near the bottom, where it gives you information about the durability of the item, etc. Hit the button.
You go into crafting mode and have the item displayed at the top for repair. Go down to the boxes and select the item you need to repair it, hit "Main Hand" and go!
The repairs are instantaneous. If you fail, it seems it only eats up whatever item you used to repair it, and does not negatively effect your weapon.
And that's it!