Honestly, finishing is a skill on it's own. The writing of an ending, and the act of ending a story.

You need to practice this, so never give up on a fic. Write and finish, even if it is shit. Every master started as a novice.

How to write an ending, there has to be a climax, a solution of the plot. I can't really help with that, each story is too unique for a fit-all-sizes advice.

But an advice piece I want to share is, after the climax, there is down time. Last threads of side plots, mainly relationship stuff, get resolved in the down time.

The down time is a thin line. It is easy to scratch it out too long or to rush it and just end the story. Some minor plot threats may not be resolved, there may not be payoff to a relationship.

Just make sure there are no open questions.

Unless you plan a sequel.

Finishing as in the act of finishing. It is a skill of its own that needs practice. Just write and don't give up.