For now we dont use key. If you get cipher text and put into your Fesitell network you should get plain text. Its not real safety. Lets add a using secret key for more security.

We should define Key size. Let's say it is equal to 32 bits. 

We are going to use a part of key on each round of Feistel network. Lets split key to 2 parts and use first part if round number is odd and second part otherwise.

Now our round function from "Create simple round function" chapter should be like:

For decryption we should use the same Feistle Network. Note the reversal of the subkey order for decryption. This is the only difference between encryption and decryption.