Learning about Feistel Cipher I'm learning about Feistel Ciphers at the moment, but having issues with the formulae, so I can't move on from it.

Encryption: 


$_{+1} = _$ 
$_{+1} = _ \oplus ( _ , _)$


For the round function $( _ , _)$, could someone advise on what exactly I need to do? Let's say my key is $1197$, what needs to happen in between the brackets, $(_ , _)$ , full diagram screenshot attached, thanks.
I guess it makes more sense if you write the two equations like this (referring to the nice diagram in Wikipedia):
$$L_{i+1}=R_i,\; R_{i+1}=L_i\oplus F_{K_i}(R_i).$$
In each round, the round function $F$ takes as input the corresponding round key $K_i$ and the previous round's right output, then computes the output value, which is then XORed with the previous round's left output. 
$F$ is usually modeled as a pseudorandom function and has many instantiations (see here for a list of Feistel ciphers, of which each specifies its own $F$ instantiation and key schedule).
This post discusses the requirements/goals for good $F$ instantiations.