So, assuming you have a very rough concept for the game and are now at the design phase, how do you actually describe how the game works?

There are really no rules for how to do this, but you can divide your theoretical game into the important core bits and think about how each one will work, what the rules are, and so on. The more information and the more specific, the better it is. If something is vague, you'll want to expand on it.

For instance, let's take combat in our hypothetical turn-based RPG.

Combatants take turns selecting actions until one team of combatants is dead.

What order do combatants fight in? How many teams are there?

Combatants are divided into two teams: the player team and the enemy team. Combatants are ordered by all players and followed by all enemies. They take turns in order to select actions until one team of combatants is dead (either the enemy team or the player team).

What sort of actions can combatants select?

Combatants are divided into two teams: the player team and the enemy team. Combatants are ordered by all players and followed by all enemies. Combatants take turns in order to select actions (either attacking a target, casting an ability, or consuming an item) until one team of combatants is dead (either the enemy team or the player team).

And so on.