These large finches have striking colours, the males are bright red and the females green. However, they can be hard to spot. Look up at the tops of conifers where they feed on cones, prising them open with their specially adapted beak.
In NPT they are best looked for in conifer plantations. You may be very lucky and see them come down to the ground to drink from a pond.
Best spot = Crynant Forest
For more information see the RSPB Crossbill page.