Or look at the PC mars website for a phone number to call or other suggestions.
If it's like other places to order checks, it will be a secure website where you will input the your name, address, & email along with the bank info & routing numbers, style of check & how many, what check number to start with and in what order to stack them in the box, and so on. They usually offer an opportunity to double check to make sure all of the information on the finished product is correct before moving you to the checkout to pay with a credit card. They will usually send an email confirmation about the order and shipping updates. A few days later you'll get a box or boxes of computer checks in the mail.
Lot's of places print checks for lots of different accounting software programs, so there could be other sources than what I've suggested.