In general there are two ways to start Smart Checkout. You can make the customer open the checkout URL directly. You can offer the URL in your web application, send it by mail, etc., etc. On the other hand you can embed some frontend code provided by secupay, and pass the Smart Transaction ID to it. It can open the checkout in a nice layer above your page.
We simply put the URL from checkout_links/url_checkout into our browser's address bar. This opens Smart Checkout fullscreen:
As you can see, it is very easy.