Flag Limited UK

Hi everyone, I'm trying to learn whmcs integration rather than splash the cash every now and then.. I've managed to successfully integrate the header & footer into their respective places, but I'm abit stuck on the next bit.. Does anyone know how I go about getting the client login box to load in the centre of the page and get the clientarea.php members area showing correctly? I think it's a crossover of the header website.css but i'm not sure..