Hi and welcome to NGA!
Hand pollination of flowers to get seeds is easy for the most part as the two bits involved are easy to identify on most flowers. Spigelia flowers on the other hand aren't so easy, however, the flowers have both the male and female parts in the same flower so you're all set to go.
The basic process is to put pollen that is produced by anthers onto the pistil which has tubes that run down into the ovaries. This is usually done with a fine tipped paintbrush. You can also use a cotton swab, but I'm not a fan of that because you run the risk of it absorbing the sticky liquid on the pistil which catches and holds the pollen while fertilization occurs which can reduce your chances of it 'taking'.
That said, what you want to do is with your paintbrush or swab, collect the pollen on the tip of the brush. In the picture, it is labeled number 1. The pollen is the yellow fluffy stuff you see. Once you have loaded the paintbrush with pollen you want to brush it onto the pistil, number 2 which is at the tip of the structure 2 is pointing at. If the hand pollination 'takes', in a week or so you will see ovaries at the base of the flower (2nd picture) start to swell. Not all hand pollination works so don't be disappointed and give up. Just keep hand pollinating every flower you see. If more than one flower is blooming and producing pollen at the same time, you can use pollen from one flower to pollinate a different flower though not necessary.
