I have recently picked up the Norwegian Defense (also known as Northsea Defense),
1. e4 g6 2. d4 Nf6.
All modern engines claim it is not good. But I have great results (14W-1D-5L in 15+10 on 2000 Lichess rating) with it and I am seriously considering adding it to my repertoire, so I would like to do some preparation work, look for critical positions where I would have to know an exact move or a plan to justify my opening.
First I have looked at games in the Lichess database. I was very surprised by Black scores in some of the variations. For example
[fen ""] 1. e4 g6 2. d4 Nf6 3. Nc3 d5 4. e5 Ne4 5. Nxe4 dxe4
scores 55% for Black in 2000+ rapid and classical on Lichess.
I have also looked at the Master database. I have noted that the character of the game is mainly set within 5 moves by White and gathered some common resulting positions. From there I wanted to work further, so I looked at master games more indepth.
Some are clearly surprised by the opening and choose a timid way to play and Black equalises before move 10. In the more critical tries White keeps the advantage for several more moves, but then lets it go or even slips into a disadvantage. Also, the games where both sides offer resistance are very rare. Very often on move 7 there are 2-4 games.
Following engine lines and picking up moves that were missed is rarely helpful, as the engine of course will mercilessly squeeze the position with inhuman precision and will disarm any initiative without effort, making any active attempts seem futile. Sometimes I do not see why White is better, so I follow the line into endgames that are slightly better, but probably considered equal at sub-master level.
Of course I analyse my own games, but the opponents crumble surprisingly fast once on their own, so the opening does not really get tested much in my own games.
So, how would one proceed to prepare in unorthodox openings apart from learning by his own games?