Simply follow the link, get the code and book. Minimum value is £10, but most of the cheapest Advances in to London start at around £12. Remember you use Railcard discount too so long as the ticket is no less than £10 after discount.
If you're hovering just under the £10 mark why not try looking at First Class? Tickets are usually around £5 more in advance so consider it a free upgrade!
Only valid on EMT services to and from the capital. Got to book off their website but since there's no booking fee it's all good :)