Binding of Isaac: Rebirth Wiki
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This page lists some useful Modding tutorials.

The Antibirth developer LeatherIceCream demonstrates how to create sprites in the style of The Binding of Isaac. Applies to all versions of the game.
A series of video tutorials by Lytebringr focused on Afterbirth+ exploring various modding topics such as XML and PNG modifications and Lua scripting.
A collection of video tutorials by Eufuu focused on Afterbirth+ that provides some basic knowledge on how to get started with modding.
A video tutorial by OnyxDarkKnight that shows how to use the Animation Editor that comes with The Binding of Isaac: Afterbirth+.
A series of video tutorials by Slugcat focused on Repentence changes about modding.