Before you begin installing, I recommend creating a backup of .minecraft/bin/minecraft.jar

Installation Using .class Files:

1) Open/Unpack minecraft.jar using an archiver such as 7zip/WinZip/WinRar
2) Delete the folder called META-INF (if it exists)
3) Insert the contents of the "classes" folder into the minecraft.jar; overwrite any existing classes.
4) Close/Repack minecraft.jar
5) Run Minecraft

Compiling .class Files from source:

1) Obtain net.minecraft.src .java files using MineCraft Coder Pack (MCP) (Version mcp723)
2) Merge changes according to instructions in Kabo/HostileSpawnOverlay.java
3) Copy CommandHostileSpawnOverlaySeed into net.minecraft.src package
4) Create a new package Kabo.HostileSpawnOverlay and copy HostileSpawnOverlay and Block Position into it.
5) Make any other desired changes to the code.
6) Recompile using MCP
7) Reobfuscate using MCP, you should now have the reobfuscated .class files which can be inserted into the minecraft.jar