is it easy to self polinate dahlias?

Check on "you tube"
. I stumbled across a video on that. Looked pretty easy. They used a little pint brush.

they dont say at which stage the dahlia is ready for polination

Maybe just watch the bees. They know! I've noticed when I collect the seeds, the outer seeds are always more mature than the ones in the center. So I think you could start pollinating the outer edge fairly early .