How to save vs. things
To makes a save you must roll under your current stat score on a d20. You may spend stat points to roll additional dice and thus additional chances to succeed. It only takes one roll to pass, but if any come up as a 20 you automatically fail. The dice can either be rolled together or sequentially, ask your GM. I prefer sequentially since it requires less decision making time, is more suspenseful, and requires the player to ask themselves exactly how important this roll is "just one more".
Unless otherwise noted, "spending" a point means reducing your stat by any number and gaining an equivalent number of extra die roll. Rolls of 1 or 20 (whichever is bad) may have consequences.
Spend charisma points in rolls defending against charms, improve reaction rolls and to ensure the loyalty of your henchmen
All damage is subtracted from your constitution. When you lose all of them you may die or become injured, depending on how your group does this kind of thing.
The only saves made against Con. should involve poison and death. The stat is overburdened, use others if you can imagine them applying. Con is only for when a very physical death is on the cards.
Used for saves involving dodging things you see coming. Also spent to help in hitting things. Much like saves, each point spent gives you another die to use, if any come up 1 then you fail and possibly even fumble
(Dex may fluster some. Without stat bonuses it doesn't give a bonus to AC any more. I say, toss out AC and use Dex instead. Dex 16? Ac 16. Spent some points to hit better? AC went down. Tough titties)
Intelligence is protection against illusions, duplicity, anything that assaults your rational faculties.
Can be spent to help in magic making by allowing additional dice. Whether 1s are automatic failures is down to the GM's tastes.
Spend to hit harder. Each point lets you roll another instance of the damage dice, picking the best (or whatever you want really). Rolling a double (triple, quadrupole...) lets you add them together, keeping them both. Rolling 1s doesn't do anything special here.
Used to save vs things you might otherwise use constitution for. Such as, sickness, blood draining, strangulation etc.
Saves vs. magic that is neither physical nor bewildering. And for anything else for that matter. When in doubt, use wisdom. If a rock is being rolled down a cliff at the party and no one noticed, use wisdom. Consider it to be luck or intuition.
Running out of a stat
CHA - when you run out of charisma just think "4am on the night bus on your way home from a party". Like that. No real downsides, but you're not much fun.
CON - Die, probably.
DEX - Everyone hits you automatically. You better have some good armour (as long as you're using my armour system, otherwise just keep whatever the armour's AC is as a minimum.)
INT - You are so tired. When you rest you MUST replenish your intelligence before all others.
STR - You're fatigued from all that killing, roll two damage dice and pick the worst when stabbing folks.
WIS - Your luck has run out. Literally. If there is ever any doubt about who is picked to be attacked, or walked into the room first when people forgot to say, or affected by an otherwise indiscriminate effect, pick someone with no wisdom left.
Recovering stat points
Every time you rest you may fully recover ONE stat. All the points, poof, they're back. If you rest somewhere safe and comfortable you may recover 3 of them.
Safe and comfortable does not include dungeons or the outside, ever. Bed + pillows + no threat of being eaten.
What about monsters?
Monsters should be interesting and unique already. Humanoids can spend their HD to represent any stat of their choice.
I generally wouldn't use this for anyone other than players because it is a lot of busy work for a bunch of NPCs and asymmetry is fun.
I should probably make a character sheet. If only to figure out what's left on it.