Go to the ".minecraft/bin" directory and backup minecraft.jar
2. Using 7zip or equally capable archive viewer open minecraft.jar
3. Delete the META-INF folder (if on a MAC *only* delete the Mojang_c files)
4. Copy ALL the ".class" files contained within the mod to minecraft.jar, replacing the existing files.
5. Close minecraft.jar
6. Copy ALL the ".jar" files contained within the mod to the ".minecraft/bin" directory (the same directory as minecraft.jar). This means copy WorldEdit.jar into ".minecraft/bin" NOT into Minecraft.jar