Open the minecraft.jar file with WinRAR or the program of your choice. (Some programs may make another .jar file with the name "minecraft" in it, such as xau's HD Texture Patch which made a "minecraft-1.1_02" jar in my folder. Don't mod these .jar's, only mod "minecraft.jar")
Then simply drag and drop the files into your Minecraft.jar, re-save and run MC.