finally 2 years later, by trial and error, i found how to do it.
of course the fronds are now fuller, and the bulblets a bit thicker too but the Woodwardia ( 5 nice fronds) is not yet mature enough to make spores.
It is better to leave the bulblet on the frond and treat it as a marcotte, which it probably does naturally.
But the production of the new fern is accelerated if I bury the bulblet in a pot of potting soil. another method according to a fern grower, is to bury the bulblets under some plant material. But according to him it takes longer than germinating spores!
however, my little bulbils, which I put in a pot of potting soil in october, are now swollen to about 3 cm, blond in colour as far as I can see, and are developing nice roots.
I cut off the ends of the fronds: they have been free for 1 week.
2 out of 3 are starting a new leaf.