Bansōkō [絆創膏] - Mod Patcher
Bansōkō [絆創膏] is a convenient tool for making changes to Minecraft mod files. This coremod works with game versions from 1.7.10 to 1.12-1.17 and significantly simplifies the patching process.
IMPORTANT: Before using, be sure to create backup copies of the target JAR files!
How to Use
To get started, create a directory named bansoukou in the .minecraft folder. You can do this manually or simply run the game with the mod installed - it will automatically create the necessary structure.
Inside the /bansoukou folder, you can organize patches as follows: each subfolder should correspond to the name of the JAR file from the mods/ directory that you want to modify.
Example folder structure:

Inside each such folder, replicate the directory structure of the target JAR file and place the files that should replace the original ones. The file paths must match the location in the archive.
Example contents of Bansoukou folder:

Corresponding structure in JAR file:

Note: To delete a file from the JAR archive, simply create an empty file with the same relative path - Bansoukou will recognize this and remove the corresponding file during execution.
Coremods: They work, although not as elegantly as I would have liked. Simply place the Bansoukou JAR file at the very beginning of your /mods folder.