I have just redeemed an iTunes voucher how do I now spend it

Just sign into the iTunes Store account in which you redeemed the voucher and buy as normal. The iTunes Store will deduct the cost from your credit balance automatically. The only thing you can't buy with such credit would be a gift for someone else.

Regards.

You redeem the code into a specific iTunes account

iTunes Store: How to redeem a code

The you just make sure the iPod is singed into that account (Settings>iTunes and App Store). Then just make purchases. Money is first deducted for the voucher balance before a credit card is charged.