When you want to increase your print credit, click the ‘Gaia’ icon in the tool tray of any Windows college computer. It’s the green icon with a circular arrow in it...

You will be shown the Gaia information box. Click the Recharge button.

You will see the Voucher Recharge dialogue. Enter the magic number from the voucher into the Voucher Code text box. Then enter the value you want to redeem from the voucher. You can redeem the entire value of the voucher or just use part of it and keep the rest for later on.

You will see a confirmation that the credit was successful.
Important:
Once all the credit of a voucher has been used, it cannot be used again. The code number is unique to that voucher and cannot be used once it has been exhausted.
Until that happens, however, the voucher is equivalent to cash. Keep it safe.
The expiry date on the voucher is generous, but keep an eye on the date. The voucher cannot be used after the expiry date.