Firstly, order your printing (letterpress, digital, foil, whatever you fancy really) in the size that you require. For instance - if you want a 120x180mm card printed white Ink one side, order that and choose your stock.
Add that to your cart.
Go to Print Finishing under The Printery tab and you will see the Die Cutting menu.
Choose the shape and size that you need relevant to the printing that you have ordered - the sizes must match up! (With the exception of 95mm Circles - for these you will need to order 100x100mm flat cards, but make sure you supply your artwork at 95x95mm).
They don't however need to be all cut from the same stock - say you have ordered 60 x diecutting, you could have this made up of 3 different designs on 3 different stocks with 20 copies of each if you wanted to - just so long as the size is the same and the quantities add up. (This is a great way to sneak in some die cut samples if you wanted to!)
Add that to your cart and you're done!
For shapes that are too big for our standard envelopes, they're ordered a little differently:
Printing by Type > Printed (larger) shapes > Select your stock in the drop-down.