Select the number of Shadow Sigs you require in the Quantity box, then update the Custom Text fields with the list of Proofs you want shadow signed, and the numbers of each. If you run out of room in the text fields, feel free to email.
E.g. If you purchased six shadow sigs, you might instruct as follows:
Gnarled Professor foils x 3, Koma's Coil Token, Hagra Constrictor reg x 2
Add Shadow Sigs
If you haven't already, you might want to check out the FAQ before you place your order. It covers stuff like what media I use to sign the cards, and how to add shadow signatures to your order.