The number of stars in the meal times 10 = HP boost (max +50)
For each fresh ingredient you get +25 stamina
Set skills never change, they'll always be attached to those combinations.
Daily skills change each cycle/quest/time you boot up the game, check veggie + veggie to see every one that normal group gets, and once you unlock drinks in port high rank drink + fish to check the special group which is more likely to have lucky cat and carver skills.
To upgrade the food stars finish offline village requests and quests, and do canteen quests in port.
Food items will usually boost one stat in a combination, meat will boost attack, grain boosts defense, fish gives oxygen (when with itself or milk), vegetable boosts element resistance, drink has different daily skills. You can only get one, you see it next to the health number (??? if haven't tried the combo yet).
Method of preparing just gives four options for each food combination, usually with different set skills, only exception being drink + drink where all set skills are unlucky cat.
That's as simple as I can make it really. Hope it helps. You'll memorize the daily combinations on your own if you just mix things up a bit. There are NO BAD COMBINATIONS ANYMORE, you won't get -50 stamina or -30 HP from anything. Just watch out for unlucky cat skill, that might minimize stamina bar or drop your HP to heroics level.
Meat + meat = two normal daily skills
Milk + vege = Bigger element resist boost but no daily
Fish + Drink (saute) = Set explorer skill
Milk + Fish = Good gathering and transporting set skills
Vege + vege = 3 normal daily skills, no set