This story has two ways of being played.
One is an rpg styled way of playing this, which means you will have to keep track of many numbers and includes rng and you will need to adapt to the situations since you don't have complete control over them. It is suggested to have a notepad file or a paper where you can take notes.
The other way to play is by ignoring the rpg elements (they will be the red lettering) and taking the complete control over any situation. That means that you will choose whenever you succeed or fail any situation and not statistics.
Now you will choose your eeveelution, each of them has different statistics which will be used in the RPG-style game and a special trait that in most cases will be used in the story to be able to do certain actions. When you are required to use an rng, you will need to generate a number between 1 and 20, then add or subtract the modifiers. Whatever you are doing succeeds if the generated number after applying the modifiers is lower than or equal to the statistic you are using, otherwise, you fail.
You are fighting with a fighting stat of 15. And you need to check with a -1(which is applied to the stat, that becomes 14.) you succeed if you generate 14 or lower, and fail if you get a 15 or higher number