It sounds like you have it right. Old seed pods will be very grey and probably have already shed. Current year's flower pods will be green and unripe. Mature seed comes from pods at least a year old, they will have begun, or shown signs of, splitting. You can pick this and tap out onto paper or collect those pods just beginning to open, place in a paper bag somewhere warm, and allow the process to continue. The seed is very fine, light brown.
Bushfires here sometimes hasten that opening process. I must say in the worst drought in 100 years recorded (and I've only been around roughly 1/2 of it) I have never seen callistemon in our area so full of flower before. Survival mechanism I guess, I just hope it means rain is coming, we are in big trouble otherwise.
For info on propagation there are probably lots of sites if you use your favourite search engine. This may get you started

Here is a link that might be useful: bottle-brush from seed