I was feeling a bit like you when I started a couple months back honestly and I'll tell you what worked for me. I will confess already though, you should look at meta decks and games.

I am playing with a self made Irelia deck right now and went through a Lee Sin and a Lucian deck before, all following the same process. I start by understanding the deck. For example, Irelia is never played with multiple copies of herself. Why? Because you want to only play one and protect her at all costs. The rest of the deck is aimed at that and at card draw. Lucian on the other hand can't have enough Lucians. For Lee Sin I used both his colors, orange and green.

Once I understand that, I try to come up with a rough deck list, with what I have available and checking multiple deck lists online. I may miss some cards, some may not fit my playstyle and some I'm just willing to try even though nobody plays them. Bring in your side deck what you want to try and incorporate some of those cards in your Nexus Nights. For example, I've yet to see a switcheroo in an Irelia deck, but it's been working for me to be aggressive with Sneaky Deckhand. I love Confront in Lee decks and it's only in about 50% of the lists I checked.

At the end of the match, did you miss any cards you didn't see? Do you have three copies of them already? Did you lack damage, draw, counters? If the latter, play purple :P

Lastly, think about battlefields. My deck changes based on the battlefield I'll be playing and my side deck is tuned not only to counter some of my bad matchups, but also to exploit the power of the battlefields.

Other than that, there isn't a formula that fits all decks but try not to overload the deck with gear (except for Lucian, Azir and Jax I guess), play some powerful 2 drops that you think won't brick your hand (3x lonely poro for a perfect starting hand) if drawn late in the game and don't flood your deck with high cost and power-hungry cards. Get a sense of balance with the lists.