If you want to save seed from your Sweet Peas bear in mind that the plant will stop flowering and put all its energy into saving seed. So earmark one plant for saving seeds. Check the variety name as you cannot save seed from any F1 varieties, if you do the flowers you get next year will be weaker.
Green seed pods will start appearing once the flowers have faded and died. Leave them to develop and when the pod starts turning brown, this is the time to remove them from the plant. Put into a paper bag, never use plastic because this will bring moisture into contact with the seeds and they will rot.
Keep in a frost-free, dark location, such as under the stairs or in a draw/cupboard – ideally in the fridge. Clearly write on the bag the name of the variety so you don’t forget. Throughout Winter check the seeds are doing fine, remove any that have gone mouldy.
The following year sow as usual.