Endings can be tricky things. You have to do a lot to make a satisfying end, especially one that comes naturally and not from the death of everyone in the party. Today I want to talk about that and some of the things you need to keep in mind.
Options Become More Limited
As stories come to their conclusion, the options players can explore become more limited. This should happen naturally, but basically as you reach the climax of the story things should be moving in such a way that going off exploring or doing random things basically becomes giving victory to the opposition.
This makes things easier for you because prepping becomes simpler when your PCs can't go running off the rails and doing whatever they want. Of course, they may do that anyhow so...
Be Prepared to Pull the Trigger
Endings put everything at higher stakes. Someone is going to win. Normally PCs are the good guys, which means the opposition are the bad guys. If the bad guys aren't stopped they'll win, because bad guys tend to be the more pro-active side of a conflict.
However, if the PCs don't stop the bad guys and nothing happens...well, it makes things awkward. So be prepared to pull the trigger. It doesn't have to turn things into a completely unwinnable situations for the PCs, but things should get worse.
Wrap Up Loose Threads
The hardest part with endings is tying up all the loose threads. Odds are you have more than you think you do. The main plot has things that need to be tied up, but so do your PCs and very possibly your NPCs. It's worth writing them down to make sure you don't miss any - and asking your players if there is anything they want to see resolved in specific.
Once you have that list, just make sure you touch on those things and resolve them. If things get resolved to a satisfying degree even the "bad ending" can be a good end.
Finally, make sure you enjoy it.
Endings can be a great time to relax with rules and rulings and just make the game fun. Yes, you need to maintain the feel and theme of the game, but unless your ending is going to involve PvP in some way there's no need to worry about balance or things like that. Make it fun. Make sure everyone has fun.
You'll have a much better ending if you do that.