If you self-pollinate your P. x 'Weser' you will not get seeds which produce plants identical to it. This is because it is an hybrid and when selfed, the seed will produce plants which will probably more closely resemble one of it's two parents (P. moranensis and P. elhersae) than itself.
If you want to propagate more P. x 'Weser' it is necessary to do this vegetatively, by leaf cuttings, as this will produce plants genetically identical to the parent plant. Seed will not produce more P. x 'Weser'.
If you still want to produce seeds from this plant, use a cocktail stick or fine paintbrush and insert this into a flower, poke about behind the stigma (this is the small apron-like pad on the upper surface of the throat of the flower). When you have picked up some pollen (yellow powder) dab this onto the stigma. Wait a few weeks, whilst the petals fall off and a seed pod forms and swells. When the seed pod turns brown and splits, shake it over white paper/card there are the seeds.
In my opinion you are likely to produce more interesting and more vigorous offspring by crossing your P. x 'Weser' with another Mexican Ping, either a species or another hybrid.
Vic