Simply add the desired certificate at SSLs.com to the cart, clicking on a little basket icon shown on the item:
Then go to the shopping cart clicking on a small icon in the right top corner:
Click on the "Checkout" button to the right:
After choosing the payment method and filling in the required details, confirm the purchase:
Once the checkout is complete, the certificate will be listed under the "Purchased certs" page in your account.
At this step, the certificate is just ordered, you then need to activate it to receive an SSL certificate for your domain from the Certificate Authority.