-The first thing you have to do is make sure you've run the ResourceExtractor and have the Isaac files extracted inside the resources folder
-Second, you are going to have to rename the "packed" folder inside:
"(SteamDirectory)\The Binding Of Isaac Rebirth\resources"
so the game uses the extracted files instead of the packed files

Note: The file we want to edit is itempools.xml, if the mod support were functioning correctly, this would go inside your mod folder, but currently, it doesn't work (pretty sure it's a bug) so we instead have to edit the one in the resouces folder.

-Find itempools.xml inside the "resources" folder and back it up.
-Go to the "tools" folder and open up the itempooleditor folder
-Right click the config file and click edit
-Go right to the bottom of this file and add your items to it under the ID "511/2/3/4 etc." (We currently cannot assign a custom ID to our items, meaning we have to use the ID the game is going to assign it, if you don't know the ID your items have been assigned to, then check in game. Open the console and type "g c51" and see check the suggestions, it should show you the ID of your custom items, this may change if you install more item mods)
-Save this file
-Open the itempooleditor program
-Click load
-Choose the itempools.xml in the resouces folder
-Now you should be able to use this program to add your items to the pools you wish (The items you added will show right at the bottom of the list on the left)
-Once finished, click save and overwrite the itempools.xml in the resources folder


Your items should now show up in the selected pools (if the mod is enabled in-game of course)






This comes with a few issues though, the first being that if you intend to pack your mod up and upload it to the workshop, it is not going to come with the itempools.xml that you created, as the workshop only uses files packed into your mod folder and as of right now, we can't put itempools.xml into the mod folder (as the game seems to ignore it and use the one in the game folder instead).

The second being that we cannot assign custom ID's to items, meaning, there is a chance that if you run multiple item mods at once their item ID's may get mixed up (as the game assigns them automatically in what seems to be a completely random order)