A4 size sheet of thick clear acetate.  The technique this time with the new kits is creating shaker frames so a sheet of this will be part of the main kit to use. If you'd like more acetate or like to follow along the shaker frame ideas with another kit you can grab a sheet of acetate here.
 
A sheet of this is part of the March/April Main kit so if you subscribe to the Main kit only order this if you'd like an extra sheet as one will already be in your kit box.