Hi welcome to GoY, I wouldn't bother trying to collect your own seed, the seeds of begonia are minute, about the size of a particle of dust, you would be better off buying the seed, and when you open the packet put in a couple of tea spoons of dry silver sand, and give the packet a good shake, so that the seed mixes with the sand, then spread the whole lot over a full size seed tray, water from below, and put the tray into a propagator at 70 deg f Derek.