I've finished stories and short novels while mostly pantsing.
The trick for me is to have goalposts.
When starting I'll either have a basic idea of what I want to accomplish, and the main characters, or I'll start with a total blank sheet.
Basic idea: I don't outline, it never works for me, and I get bored. Instead I'll have a basic plot with some features I want to show. It could be a theme like, 'be true to yourself', 'family matters', 'betrayal', etc. It could be a setting, a fantasy hive city in a living city that uses steam punk features and will focus on the sewers where the criminals and lowest of the low make a living. It could be the character, with a large backstory and they need to do something. Or a scene, like a climatic cat and mouse fight in a series of labyrinthine tunnels between several humans and an overpowered monster.
Vague enough to point me in the right direction, while still leaving me lots of room to play.
Then I come up with some goalposts.
1st goalpost: Bob the sewer cleaner finds something in the sewer that is really important.
2nd goalpost: Bob the sewer cleaner is running from government enforcers who want the really important thing, and is rescued by rebels.
3rd goalpost: rebels are betrayed, Bob and co. need to flee, keeping the important thing away from the government.
4th goalpost: Someone close to Bob dies.
5th goalpost: climatic fight, the important thing does something that breaks the governments hold on the city.
What is the important thing? No idea. I'll figure it out as I write.
How big are the rebels? I'll work on it later.
Who are Bob and co.? I'll throw stuff at the board and see who seems interesting enough to add to Bob's group.
Who dies? Who has become the most important person to Bob, they're going to die.
Climatic fight? I want Bob to be bloodied, battered and half dead by the end. I can worry about the details when I get there.
As I write I'll give myself some more goalposts, and alter ones I've already written.
Maybe I like one side character so Bob starts a relationship with them. The first kiss will become a goalpost to aim for.
My muse decided that having a giant stone figure at the heart of the city is cool. It's a religious symbol and legend has it that it helped build the city. Cool the important thing is the key to turning it on and controlling it. The climactic fight will be Bob and the rebels trying to reach the statue figure out how to turn it on and using it to overthrow the evil dictatorship. New goalposts they need to learn they have the key. They need to rally the rebels and malcontents. And they need to fight the battle.
I get lots of freedom to pants things but I always have a target that is reachable and will keep me motivated.
Blank Page: I used to do this a lot. I also have a lot of old stories that never got finished.
With the blank page, I just start writing. As I go along I spot things I can use, interesting side characters, little setting details, the MC's personality, etc. I turn these into goalposts or plot.
So, I have Cindy she lives in a giant snail and goes from town to town selling herbs she picks up along the way.
She really loves her giant snail, and doesn't interact well with people.
Goalpost: She needs to lose the snail.
Goalpost: She needs to find someone who can get her to open up.
She is travelling around the edge of a kingdom, close to a jungle full of monsters and savages.
Plot: Soldiers confiscated her snail to help move supplies to an army that is working at pacifying the jungle.
One of the people Cindy interacted with near the start interests me. As she's following the soldiers trying to think of how she can get her snail back, she meets up with the person who wants to.... get their family member back after they were conscripted.
Goalpost: Find snail and family member.
Goalpost: Romantic moment between Cindy and Pat? Or very heart to heart platonic talk?
By coming up with goalposts and plot points as I go along, it again gives me direction and small goals to let me know I'm accomplishing things.
Without the goalposts, you won't know when you're done, and things just keep going on and on, with no end in sight. You can run out of steam when the ending seems SO far away. And you don't know what the main plot is, so you fumble from one thing to another without anything really tying it all together.