BizHawk or Mesen S emulator
Download the appropriate item_editor.lua script from the releases page.
In BizHawk or Mesen S, open the Lua Console and load item_editor.lua.
If there are no errors in the lua console, it should be working as expected.
While on the main playing screen or the item selection screen, press the L and R shoulder buttons1 (as configured for your in-game SNES controller) simultaneously to open the item editting window. If you were not already on the in-game item menu screen, it will open (to pause the game while you edit items). This will show a list of items that can be set. To switch between pages of the menu, use the L and R shoulder buttons. To move between items on a page, use Up and Down. To edit an item, first press A then Left/Right to change values, and either A or B to confirm. To exit the editor menu, simultaneously press the L and R shoulder buttons again.
While the editor menu is open, input will be swallowed by the lua script and will not affect the running game.
1This does conflict with quickswap if you have it enabled, but I don't have any better ideas. Feel free to suggest an alternative if you have an idea!