No need to get complicated, no need to get overly detailed.
I use a pack of index cards and tape them to a blank wall or the fridge. The first card has "Introduce the protag and the problem" of course.
For the rest, write or type (I have an old electric that serves well) a 1-sentence description of each scene. "John and Buford disagree about ____, knock down fight happens, irate neighbor calls cops."
Books are composed of scenes, each with a beginning, middle, and end, and each scene
must be necessary to the plot. Limit it to one scene per card. If you include too much detail it can bog down and take all the fun out of the actual writing.
Be aware that most outlines never survive intact. That's normal!
As you develop the plot, new subplots and scene ideas will occur to you. Put those on cards, and shift the older ones to make room. If some scenes become redundant, remove the card.
For practice, outline a favorite book in this way.
Card #1: Hap drives up while Leonard is celebrating Christmas Eve by setting fire to the crack house next door. This is the third year straight. The "Ballad of Davy Crockett" is playing in Leonard's house.
Card#2: Police and fire arrive. Darkly humorous observations are made between sheriff and Hap about Leonard's idea of throwing a party.
(
The Two Bear Mambo, by Joe R. Lansdale [Bubba Ho Tep])
I also recommend
Save the Cat, which is about screenplay writing, but adapts to novels easily enough. It has online index cards to help with plotting. I prefer using real ones. Those are harder to delete.
One good reason to outline is to help with the pacing. Two to three scenes strung together make a decent sized chapter, 12 chapters or so make a book.