Give the sales team a call with your order number handy. They will confirm all your personal details and your purchase and then they need to do a credit check/approve you for the loan.
If successful, they will send you the documents via e-mail. Print them and send them off, simple as that.
I've just done this myself as I couldn't afford it outright and my laptop is just about to die.
I spoke to a lovely lady who even threw in a free carry case and flash drive worth nearly £90.