Choose your Moment
A session should usually end at the end of a scene (unless you're doing a cliffhanger ending). Let the final scene reach its natural conclusion then tell your players "That's all for tonight. Thank you for playing." You don't need to use those exact words, but the idea is to have a firm statement that the action is over, and a positive acknowledgment of the players' involvement.
Where does the scene end?
As a new RPer, this might not be clear right away (if it is, just skip ahead to "More precise timing"). A scene is a chunk of your narrative, one of the "sub-stories" in your game session. A scene will be a sequence of actions that are tightly connected.
Think about scenes in movies. It's mostly the same. A scene change is often when characters move to a new location, or start doing a different activity. Here's an example of scenes a fantasy adventure session might have:
- Figure out how to open the secret cave-dungeon door
- Travel through the first few corridors of the dungeon, avoiding the
- Run past the cave troll before it grabs you
- Find & Kill the dragon; it was evil
In this example, most of the scene changes happen because the characters are going to a new place, but the important part is that the characters are doing something distinct in each scene. Scene 2 could have spanned multiple rooms and tunnels, but the action is basically the same throughout.
More precise timing
Once the major action in the final scene ends, when do you say "That's all"? Immediately after you narrate the dragon's death (or whatever the important action is) is one place you can end things. "...its breathing stops and the room grows silent. That's all for now, thank you for playing."
This is slightly abrupt. That's a good thing if you want to underscore a major event that just happened. The jarring end will leave players with a sense that something big just happened. The aftermath of the event matters, but we don't get to see it yet. Players will leave the game with the final moment etched in their minds.
Another option is to let characters "clean up" at the end. Usually, characters want to do things like exchange a little dialog, search the bodies, or check each other for injuries after some major dramatic sequence has ended. For a less jarring ending, let your players do this.
Listen carefully to their actions to pick that right moment to end things. They will probably have a few things they want to say right away ("My character claims a dragon tooth trophy", "Sally, are you hurt?!"). Let them say it, and resolve the immediate actions. Then, declare the end of the session. Be careful not to let the players keep going too long here.
Rewards & Post-Game
Immediately after you declare the end of the session, start the post-game rewards & wrap-up. In most systems* the GM is going to do all the talking for the rewards portion, so go straight into the post-game Q&A when you're ready.
That's all for tonight. Thank you for playing.
You each get 5xp for slaying the dragon. Bob, you get 1 bonus xp for
excellent roleplaying. Sara, you get 1 bonus for landing the deciding
So, what parts where the most fun that session? [GM-led discussion]
Are there parts you would like to have done differently, or didn't enjoy? [GM-led discussion]
*Some games, like Mouse Guard, have the entire group discuss and decide how to assign rewards. Post-game works basically the same there, except the GM is acting as a discussion facilitator rather than monologuing.