I've used several methods, all of which were successful. The simplest was to gather the fruits in March after overwintering under the parent tree. The fruits were soaked in moderately chlorinated water until soft, then manipulated by hand until the seeds separated from the pulp. At this point the seeds were very thoroughly, repeatedly rinsed.
The seeds were sown into a seedbed of finely prepared soil amended with perlite in the upper 2 inches. The seeds were evenly sown on the surface. The seeds were covered with an inch of clean sand to prevent crusting and prevent weed germination.
By planting at adequate density, the seedlings will grow tall with no low branching, making budding/grafting considerable faster and easier.
Upon reaching approximately a foot of height the seedlings were individually potted into deep pots. The potting of the plants should not be performed while the plants are flushing, to prevent stunting/wilting.
I bought seeds on ebay from two different vendors, put some on wet paper and some in a coconut fiber container near a heat source because it was in winter. I don't know if it was the time or the method, but it was a failure, neither was born. Later I received fresh yuzu and sudachi seeds, and I tried again with the coconut fiber and the heat source, just as I managed to germinate one.
where I scattered seeds anyway, some of which were not even completely buried. After a few months of leaving them outside in the shade, in May/June many poncirus, yuzu and sudachi have sprouted. The only problem I had was that the snails or slugs devoured a few until I saw what was happening.
I just wash the seeds in a colander with dish soap and rise them good, then plant a smattering of seeds about an inch to half inch in small pots of miracle grow potting soil, Cover each pot with plastic cling wrap and poke holes in it to vent, and set them in a south facing window in winter/ after harvesting fruit and making marmalade. Almost every seed sprouts within a month, having watered the pots once a week.