"See the Player's Handbook." But it's 3e-style multiclassing with AD&D style dual-class stat requirements. Maybe Plate and Shield not worth it for Wizards anyway, probably less cost to just grab 20 Dex and concentrate a half-decent buff.
Part 2: Playing the Game
Chapter 7:Using Ability Scores
Well, actually it's the meat of their game rules, not much to do with ability scores other than grabbing the mod, which you already needed for Chapter 2 and 3 and 5 anyway. To quoth once more: "Fucking game manuals! How do they work?"
Advantage/Disadvantage. AKA the only thing they actually designed in the last three years. Handy that the thing where you never use rerolls because they're all advantage actually means you only reroll one of your advantage dice. That's awful.
Your mods are +3 to +5 for stat (+5 by 8th level anyway), and +2 to +6 for proficiency if you've got your tools or don't need them or have more cake. So you need a 4+ at high level and a 10+ at 1st level. Let's say you always need a 10+ because the DM will fuck with you at high level if you bother getting good, by making you use a shit stat or something even if everyone else gets it easy.
Advantage turns that 55% to ~80%. Disadvantage turns it to ~30%. Note that's +/-5 in the old terms, and +10 for the two steps. Never anything else, there's no +2 for synergy or aiding, and there's no +10 for having exactly the right tool. It's just +5. Or -5. And they're everywhere. And they just about all depend on the DM agreeing with you, or liking your improv, or ... well, spellcasters and rogues get them just for breathing, as does anyone with a helper.
But the whole game is getting advantage, and dishing out disadvantage. It's bigger than the change in your total stat and class mod between level 1 and level 20. And you get that at 1st level with cake and blowjobs.
Anyway. With group stealth, you're all stealthy if half of you are. With group climbing you can all climb as long as half of you can. No worries for the Centaur after all. Clip-clop past the guards and up the wall, because someone else made their checks. That's great, everyone have some cake. All trying to swim? Iron Golems can if they're on the same team as a dolphin! Actually, they probably can anyway, it's all pretty loose.
So there's "skills" where you get your proficiency bonus, and "other ability checks" where you don't and may as well not bother trying. Unless you bought cake. The skills themselves are few and only overlap a moderate amount, and it looks fairly predictable for what you could ask to use and get away with. Realistically groups are going to quickly learn which characters do which jobs and never vary from that, and if you're missing a skill you can probably just use a different one without any trouble at all. Unless it's listed as an ability check and then fuck you.
For saves I obviously want Con and Wis, because they didn't really bother with the whole six-save thing even though it is in the game. Uh, one less reason to play a Rogue. Though again, advantage is the thing to seek. They hint the DM will instead just give modifiers rather than advantage, which seems to defeat the purpose, but whatever. I guess that's more of the Oberoni save.
Edit: Oh gods, I almost missed the downtime rules. You can heal in an hour using class-based healing dice (1 per level), and free heal to max and regain half your healing dice each night. So adventure every 2nd day. Unless poisoned, that's a random chance to recover for each 3 days rest.
If you have a year to spare (someone must be very poisoned), you can craft one suit of plate armour (lol, no), or learn a new tool proficiency. Oh, yeh, tools. They look to be mostly just used to allow various activities that aren't technically skills, or sometimes are skills, it's kinda random.
With a year of gaming, your character could learn the d20 and play 5e with his proficiency bonus. Handy.