1) Log in to quidco
2) Through quidco, go on Vistaprint.co.uk
3) Click on the link to get a free t-shirt (which you pay 2.64 p&p for)
4) Customise and buy a t-shirt
5) Wait for it to arrive
*I don't think the voucher attached will work alongside quidco, but if you don't have quidco, you can get 90% off with it.