Root is a fairly complex game. None of the factions on their own are too difficult, but since they all have their own rules it adds up to a lot.
When teaching, I like to start with the rules that are common to everyone. You win the game by being the first to 30 VP. You can earn VP by crafting items, removing cardboard pieces in battle, and in a third way that is unique to each faction. Then, explain how ruling clearings and movement works, followed by battle, then crafting.
Once everyone understands the common rules, do a quick overview of the factions and then start the game. Each player just needs to follow the steps on their board on their turn. They probably won’t get the best strategy right away, but that’s ok in a learning game. I think the walkthrough is helpful.
And Root is definitely better at higher player counts because it opens up a lot of interesting interactions between factions. But, for learning the mechanics, a 2 player cat/bird game is solid.