-They ask me to create a "secret" sentence (I don't remember in which step sorry but it wasn't in the tutorial I think. except this, the tutorial was gratand really well explained)
Electrum does indeed generate a 12-word secret seed phrase, that's the back up for your wallet. The seed phrase creation part of the process is beyond the scope of my tutorial, which focuses on verifying the PGP signatures. If you need help using Electrum there is plenty of information in this sub-forum, or feel free to create another thread to ask your specific questions.