For anyone who is unsure as to when geranium seed is ready to collect then please look at this photo. Notes that the central stamen/Cranesbill is black and that the seeds have lifted from the centre and can be seen at a 90 degree angle to the plant. Usually the seed lies at a 180 degree angle. If you see the seed at this stage or at a stage where the seed is starting to lift then collect. You can also collect the seed if the central stamens and seeds are brown but the seed has yet to lift.
Place seed in a sunny place to dry out before storing in a cool, dark,cool place.