Superfloss. At your pharmacy you will find a special floss that has a hardened end which slips under the bonded piece, between your teeth. The dentist who placed this retainer should have explained this to you.
Options. First call your Orthodontic Specialist for the ideal solution customized to your needs. Discuss floss threader, proxi-brush, Air-Flosser, Water-Pik, and other modalities. Your Orthodontist will know the best solution for you.
Floss threader. To floss under a permanent retainer you can usually use whats called a floss threader (sold at most drug stores). You can thread floss onto it, same way you thread a needle and use it to get under the retainer.