Step 1: Download BON and JDGui (a Java decompiler).
Step 2: Download and install Minecraft Coder Pack for the version of the mod you're trying to decompile. If you used Forge in your mod, be sure to install the Forge source as well.
Step 3: Double-click on the "recompile.bat" file in your MCP folder (for Windows), and wait for it to finish.
Step 4: Open up BON. For the 'input file', select your mod. For the 'output file', select the path where BON will place your deobfuscated classes, and give a name for the file. For the 'MCP folder', just select your newly created MCP root folder. You can leave everything else the same. Hit the 'go' button.
Step 5: Go into the folder you selected for your 'output file'. You should have a new file in there. Right-click the file, click 'rename', and add ".zip" to the end. It should turn into a .zip, which you can open.
Step 6: Open up JDGui, click 'file' in the top-left corner, click on 'open file', and browse for the 'output file' BON created.
Step 7: You should see all the packages and source code of your mod in JDGui. Click 'file' again, select 'save all sources', select a folder, and give a name for the .zip file JDGui will save all your sources to.
Step 8: That's it! You can now extract all the source files from the .zip JDGui created, and put them into Eclipse for editing.
Thanks for reading. I hope you learned something from this !