The seeds/plants we get of this crossing is called F1 which means first generation.

Mendel's law of genetics tells that plants with single flowers has a dominant gene, while plants with filled flowers has a recessive gene.

It means that when our F1 plants inherit one single flowering gene from the father and one filled flowering gene from the mother, then our F1 plants will have single flowers (=the dominant gene).

We now have F1 flowers with both pistils and stamens with pollen and at the same time they have a filled gene, and we can go on working with these.
We can self pollinate them or cross them with each other, and the plants we get out of this is called F2 (second generation), and it is in this F2 plants the filled flowers will occur. It is said 10% in F2 will have filled flowers, but it can change.
Real good F1 plants can by self pollination give more than 50% of filled flowers.
A such plant is my no. 29 as you see here.

Another way is to take pollen from an F1 plants and pollinate another plant in one of the four groups mentioned above (Nidan, Nichirin, Chyouji or Karako), which, other things being equal, should give even more chance to have filled flowers.
The principle is to fill as many filled genes in the mother plant as possible.

As mentioned before the main rule is that F1 plants will have  single flowers - but no rule without excptions, and somtimes you can have filled flowers in first generation. This is due to a double up of genes by nature, and our F1 plants can inherit more genes from both the mother and the father. The more genes, the more unpredictable and exciting is the result - depending how lucky we are that more filled genes will hit each other.
Be aware Hepatica must flower three years before the flower has it's final appearance
A few examples of my own producttion:

NOBILIS:
Of course the procedure with F1 and F2 plants and filled genes from the parents are the same what ever we talk japonica or nobilis, but unfortunately nobilis is not as variable as japonica and it is difficult to find nobilis types with either have the female or the male fertilization bodies and at the same time have filled genes.
E.g. the four types mentioned as possible mother plants in japonica does'n exist in nobilis.
Fortunately we have some of the filled formes in nobilis which produce anthers and pollen.
Here you see some examples:

Producing of F1 plants must because of this happens in the uppersite way of japonica. The single flowering plant must be the mother plant while the father plant has the filled gene.
It can be difficult to get the pollen out of the anthers, you must (with a scalpel, cut an incision in both side of the anther, place the plant warm and the pollen will come out.
Here are a few of my nobilis from seed:

JAPONICA X NOBILIS:
I have done some pollination with japonica and nobilis as well. My intention was to grow some plants as exciting to look at as japonica and as frost hardy as nobilis, and both succeeded.

Most of the crossings between japonica and nobilis (or nobilis x japonica) are fertile even a few are sterile.

A good advice:
When you start this crossing work it's very importent to do some registation of the every plant - mother, father etc. It's a shame when you after 6 or maybe 8 years have real good filled flowers, and then you can't remember the "roots" of the plant - and you must start all over again.

Breeding and procuce your own crossings are incredible exciting. To choose the parents and imagine what comes out of the relationship, I think talkes to a very big part of plant people. That the result not always match the expectations we must just accept.
Contrary sometimes you can be lucky or maybe skilled so the result far exceeds expectations.