Hello,
Its very easy.
After the flower fades there will develop a seed pod.
Leave them alone till they dry on the plant.
Its easy as that.
Hope this helps
Cathy

Just be patient. The pod will start to open and then you know they are ready. It's a lot of fun to collect seeds from platy.

I'm so glad I happened upon this! I love my platys, and they seem to have stopped blooming. Can't wait to find and sow some seeds!