It probably helps if you have worked for another in the same capacity and thus made contacts who know you and your reputation first.
Aside from that maybe target potential customers with mailshots/fliers to try to get your name known ? Get yourself a website and listings in the trade, business and BT phonebooks. Talk to friends and ex-colleagues to spread the word. You need high visibility at low cost to start with until you have a number of regular customers and repeat business.