Hi guys,
I'd like first to thank you all for your good advice, here is how the first pool cession went.
First mistake I've made was not to bring my mask, I brought instead goggles, I had to pinch my nose all the time. Second mistake was not bringing some weight with me, I ended holding the lower part of the pool stairs to stay on bottom.
The first half hour I was doing circle but they where blowing apart a few centimeter after leaving my lips.
The second half hour I've finaly made it. All my bubles where growing into circle and reaching surface. The most important part was the shape of my lips, the smaller and pointy shape the more successfull where the buble. Blowing some air from my lung made also the circle bigger.
I don't know if at sea it will be as easy as the pool since there should be no current at all or the bubble will go into pieces.
Thanks again