Peas are almost entirely self pollinating, only very occasionally crossing with other plants. Set aside a section of row that is entirely for seed production, and make sure you sow at a time that will avoid pea moth To avoid physical mixing up of the seeds, separate different varieties of pea with another crop. Check the row from time to time as the peas grow, and pull up any plants that are weak or not true to type.
Let the peas mature until the pods are brown and the seeds start to rattle. If the weather is very bad, pull up the whole plants and bring inside (for example hung upside down from the shed roof) once the pods start to wither, to ripen and dry further. Once the pods are really dry, shell the peas out. Dry the shelled peas further in a warm (but not hot) place, label with the variety and date, and store.
Thank you to DIY Gardening & Better Living for this information on growing and saving seeds.