RULES
Rule 0: The DM is the final adjudicator of what occurs within the game world. They are empowered to alter the strict letter of the rules, and encouraged to use common sense and the intentions of the rules to inform their rulings, as circumstances demand to make the most internally consist within the game world.
Rule 1: An unmodified roll of "20" on the d20, a.k.a a "natural [or "nat"] 20," is an automatic success. No modifiers need be applied. The attempted action of the roll succeeds. By the same token, an unmodified "1" on the d20 is an automatic failure. No modifiers can turn a "natural 1" into a success. The attempted action of the roll fails.
Make an Ability or Skill Check/Roll
Skill Rolls: Make a d20 roll. Add the skill's relevant ability modifiers/penalties and any other modifiers from class or species (such as a Thief's Skill Expertise or a Halfling's Affable Charm to a persuasion roll) that apply. If the total is equal to or higher than the Difficulty Challenge (DC) of the skill roll, as set by the DM, then the roll succeeds.
Make a Saving Throw/Save Roll
Save Rolls: Make a d20 roll. Add the relevant ability modifiers/penalties and any other modifiers (such as a Cleric's Pillar of Faith or a Half-Elf's Sylvan Sensitivity to overcome a dryad's charms) that apply. If the total is equal to or higher than the Difficulty Challenge (DC) of the skill roll, as set by the DM, then the roll succeeds.
COMBAT
The Combat Round
Initiative: Each side rolls a d10. Individuals each add your Dexterity modifier. Add in any other relevant modifiers, as from species or magic items/spells, etc... anything that would boost your character's speed to act. Numbers are placed in order, with the highest going first.Then the second highest, and so on. Enemy initiative is taken all together based on a single roll by the DM with appropriate modifiers. In the event of multiple different types of creatures/enemies attacking together, each different type of creature will take their turns as a collective group on their own initiative roll.
Ex. Jerry rolls his d10 for the party side and gets a 4. Everyone adds in their modifiers. Jerry has a 5. Bob 8. Carol stays at 4. After double checking all mod's, Barb has a 10!
The DM rolls a d10 for the five goblins and gets a 5. Goblins receive a Dex. mod. of +2 for a total of 7. The DM rolls again, d10, for the 2 Bugbears joining them. The result is only a 3.
So the round's initiative is: Barb (10), Bob (8), Goblins (7), Jerry (5), Carol (4), and the Bugbears (3) go last.
It is left as a matter of personal preference if the group wishes to roll initiative for each round separately. Typically, in the interests of engaging all players as much as possible into the workings of the game, a different player should roll initiative each round.
Alternately, the first initiative order is just repeated round after round until combat is complete.
Movement: Movements within the combat round can be taken at any time during your turn: before an attack, after an attack, both before and after, as your sole action, whatever the player wants, so long as they not exceed the distance of their full movement rate.
Missiles: Any prepared/loaded missile weapons (a nocked arrow in a bow, a loaded crossbow, a throwing weapon in hand, etc...) can be fired/attack first before any other movement and regardless of initiative order. In the event the weapon is a pulled bow or thrown object on the character's person, they can then make a second ranged attack of the same kin on their initiative turn.
Ex. Jerry's Ranger had his short bow nocked with an arrow prior to this encounter beginning. This action was declared by Jerry some time prior to this initiative roll.
Even though Jerry's character is fourth in the initiative order, his ranger can aim and fire that arrow before anyone else takes action. The ranger is then able to pull and nock a second arrow to use on his turn within the initiative order. In other word, Jerry gets to fire his bow twice in that single round.
Certain weapons, like heavy crossbows, require too much reloading time for this to occur. However a loaded light crossbow could still fire at the beginning of the round, using the character's actual turn within the initiative order to reload, and be ready to fire again at the beginning of initiative the following round.
Magic: Anyone casting a spell in a combat round must declare they are casting at the beginning of the round. Spell casting requires some degree of time, concentration, calling upon the energies you command, perform the proper incantation, and so on. You must declare your intention to cast a spell at the beginning of the round and the spell is completed/takes effect during your turn.
Melee: Once spells have been declared and missiles have been fired it's time for the movements and hand-to-hand combat to begin. The DM should go character to character down the initiative order, enacting their actions, movements, and the results of any attacks.
--Attack Roll: Roll a d20. Add your relevant ability modifier (usually Str. for hand-held weapons, Dex. for missile weapons, Int. or Wis. for spells) and any other applicable modifiers or penalties, eg.g. magic weapons bonuses, weapon or attack expertise attributed from your species or class (such as a ranger's Favored Enemies or dwarf's Ancestral Foes), situational modifiers (as stipulated by the DM), and so on.
If the total of that roll is equal to or higher than the target AC, then the attack is successful and damage is rolled.
That's it for the basics, I guess...
Anything crucial for play's operation I'm forgetting/blanking on right now?
How 'bout Morale rules? No morale rules? Optional Morale rules, I'm leaning/thinking...