Outline of the entire story? If you are bothering with an outline, and you don't want to just make up the plot as you go, then make sure to have an ending in mind for your story, and build everything towards that ending. Perhaps along the way, you'll find a different ending that better fits what you've built so far in your outline/actual story, but having an ending in mind helps a lot with getting rid of plot holes and extraneous characters, dialogue, descriptions, etc., etc.
The way I go about outlining up to the ending is building great filters, things that are definitely needed in the story and all important to the plot/how I want the story built if it's a more slice of life type thing.
To me, it's like imagining a stream of ideas flowing down through a filter that only allows certain ideas to go through that abide by things like, "something must happen like X," or "character Y must have this characteristic," and everything else of how the story builds up to that or what happens after that is left for later examination and filtration.
As I finish building the more important filters that make up the skeletal structure of my story, I start to roughly divide up the work into sections of what the beginning is like and the rest. If you are shaping your story to match with your ending, keeping the beginning and the rest of the story fluid and easily changeable is vital for easily keeping your story wholistic instead of as just different parts noticeably tied together. Vice versa if you have a beginning/middle in mind that you absolutely must have in the story and/or you have already written parts of the story.
Then to narrow down what you want to write, just keep constructing filters in a way that is coherent with the rest of your filters, prioritizing what you want the most in your story so that you don't end up having a conflicting filter of lesser importance that you've already started to write a story around when you have come up with a more important filter.
And all of this can apply not just to writing the story as a whole, but to individual chapters as well. Basically, this methodology scales really well.
But make sure not to get too caught up in the little details of the story, and make sure not to get too caught up in the greater plot of the whole, changing the plot as you like, forcing you to rewrite the details of your story a lot. Don't miss the forest for the trees, and don't miss the trees for the forest.
Also, if you can't just seem to get a good idea out into a fully written story well, just write multiple differing summaries for the ideas for chapters you are writing and the story as a whole, and expand upon a summary you think is good into a full chapter/story.
This is the advice I have for you. I hope it's helpful.
And to directly answer your question: If I feel that I only need a vague idea of what I want in order to write, then I do so, but if I feel that I need an entire plan, then I go into as much detail as I feel I need in order to connect the dots and start writing the story.