I wouldn’t say I’m sure, as it is indeed confusing, but here’s what I think is more accurate:

1. You can use 128, 160, 192, 224 or 256 bits of entropy to start with. We then add 1 bit of checksum for every 32 bits, so 4, 5, 6, 7 and 8 bits, respectively.

2. The salt input for PBKDF2 will always contain “mnemonic”, but you could additionally append a password to it (which we usually call a passphrase). So if your passphrase is “SATOSHI”, the salt will be “mnemonicSATOSHI”.

3. PBKDF2 will run the HMAC-SHA512 algorithm 2048 times and then return the 512 bit seed.

4. The seed is then fed into the HMAC-SHA512, along with the “Bitcoin seed” salt. This gives us the 512 bit master extended private key.

5.The LEFT/FIRST half of the master extended private key is the master private key and the right/second half is the chain code which we will use to derive more keys.