I am requesting the steps for casting a spell (for examples: Light for typical, Ray of Frost for combative, and Mage Armor for self).
Step 1a: if you're a prepared caster, like a Wizard, make sure you memorized the spell you want to cast.
Step 1b: if you're a spontaneous caster, like a Sorcerer, make sure the spell you want to cast is one of your spells known.
Step 2: make sure you have all necessary components of the spell: material and/or focus components, the ability to speak verbal components, and the ability to move unrestricted for somatic components.
Step 3: spend the required action to cast the spell. Some are as quick as a swift or immediate action, some are as long as 10 minutes. Most are just a single Standard Action.
Step 4: if you are threatened in combat (as in, an enemy could attack you with a melee weapon at the point you begin casting your spell), you have a choice: cast the spell defensively, and risk losing the spell if you fail, OR cast the spell and risk an attack of opportunity from said opponent.
Casting defensively requires a d20 roll called a "Concentration Check". You add your Caster Level and primary Casting Ability Modifier to this d20 roll. If your total roll meets or exceeds 15+(2xSpellLevel) then you successfully complete your spell.
If you instead chose to risk getting hit, and you're damaged, you must make a Concentration Check anyways, but this time the DC is 10+Damage+SpellLevel. If your total roll meets or exceeds that, then you successfully complete your spell.
There are other times Concentration is required, such as if you're grappled, entangled, under water and whatnot, but your GM will inform you if any check is needed should the situation arise.
Normally you don't have to roll anything.
Except for...- touch spells and ranged touch spells (d20 + DEX + other bonuses vs touch AC)
- area damage spells (roll damage, then enemy rolls their saving throw, usually for half damage)
- a variety of other spells that have variable effects (mirror image numbers, teleport error percentage, level checks for dispelling, etc)
Although of course there are many spells that don't require rolls at all, assuming you cast while safe from enemies.
IMHO, it would have been a great balancing factor for magic if spellcasters had to make a "spell success roll" for spells that got harder when casting higher-level spells, with increasingly greater consequences for failure. It would make spellcasters think twice before throwing any spell they didn't absolutely need to. But DD3.x and PF doesn't use that kind of rule, and I suspect very, very few players would be ready to accept such a houserule. Like the sanity cost of nearly all spellcasting in Call of Cthulhu... <evil grin>