I've never collected yellow flag seeds, so can't help you there. With sweet peas, leave the pods to go brown and hard. Cut them off with a little stem attached or whole stalks with lots attached, depending on how many seeds you want. Put these somewhere warm and dry either in a paper bag or on some newspaper and leave them to pop open by themselves. If you can't wait then gently twist the pods with your hands and they should open up revealing the seeds. These should also be left in the same conditions for a day or so to make sure they are dry. Opinion differs as to how to store them. I keep them in an envelope. Keep them somewhere cool to store them. To sow them, put some multi purpose compost into a 5" flowerpot, gently firm it down using a similar sized pot so that the compost is about 1" from the top of the rim. Push about 5 seeds equally spaced into the compost using your finger. Push down to the first knuckle. Make sure the seeds are covered over again then stand the pot in a tray of water for half an hour. After this time empty out any water left in the tray. Put the pot inside a large freezer bag or similar and tie with a twist tie. Put the pot on a warm windowsill. Check it every day and as soon as you see green shoots growing take it out of the bag. It will probably want watering again now. Keep it somewhere light but not too hot - not a south facing window. Everytime the compost feels dry when you push your finger into it water it as before. When the stalks are 3-4" high and have 3-4 leaves use your finger and thumb to pinch out the top bit of growth down to the topmost leaf. This will make the plant grow new stalks which will mean bushier plants which will mean more flowers next year. Look after your plants all winter, making sure they are never sitting in water for long and plant them out in the spring.

Thanks 'pond'

So do you mean that as soon as the seeds split out of the pod i leave the seeds in the bag too for a day or so to dry out and then plant immediately whether that be september, october, etc?

No, actually if you were to sow them now you would only need them to be ripe, ie dry in the pod. The reason you might dry and store them is if you were going to wait until the spring to sow them. Sometimes its good to sow now and in spring as you get a longer flowering period.