Mattress encasements generally come with specific installation instructions, but most have a zipper to make application easier. Generally, a mattress encasement is installed on a bare mattress that has been stripped of all bedding and additional mattress pads, toppers, or other accessories. You'll probably have to lift your mattress in order to get the encasement on correctly. Follow the instructions for your specific mattress encasement in order to install it properly.