Minecraft META-INF File

I have been watching videos about installing mods and Optifine on Minecraft. Out of all the videos I've watched, they always delete the META-INF File. I know what happens if you don't delete it but what if I want to add another mod into my Minecraft? What will I have to do if the file has been already deleted? Do I just install the mod and put together the mod files with Minecraft using WinRAR when I don't have the file anymore? Or will there always be a META-INF file to delete each time I install a mod?

If you dont understand what I just said heres what I'm basically saying. How do I install another mod when I've already deleted the META-INF file when there is nothing to delete. Will there be another META-INF file to delete again?