Ok, thanks for the answers. So, to sum-up here is my plan:

1) Generate (and carefully save) a BIP39 mnemonic code (offline) using this:
https://github.com/trezor/python-mnemonic
Indeed, Electrum wallet doesn't generate Bip39 mnemonic and i would prefer
to have one.
I tested the few lines of code and it worked.
Still there is something i don't understand:
entropy = mnemo.to_entropy(words)
I guess the aim of this line is to check the randomness of the list of words.
However, i got this:
>>> entropy = mnemo.to_entropy(words)
>>> entropy
bytearray(b'\x18\xe2\xa9=\x82\xd4$\x915D\x98....................................\xb4\x7f\x02\x90f\xdf\x8b')
>>>
How this should be understood?

P.S.: I read that a 12 words list is enough and a 24 words list do not improve safety. So i will
go for a 12 words list.

2) Create a new (offline) wallet by importing the mnemonic into Electrum. Generate a "receiving" address

3) Send the BTC from the exchanges to this address.

Am i doing something wrong?

Cheers:)