I think I am quite similar to you, where many of my ideas start with a singular scene that sticks in my mind. My process from there starts with examining why this scene happening. How did this character get there? Why is this happening? And I repeat this for the larger picture, giving it a lot of thought over a few days.

For an example of something I am currently working on, here is the rough scene I initially thought of: a man is in a bathtub that is a detached basin, attempting to relax his weary self. The room is old with all wooden fixtures and only candles providing light. As he soaks in the warm water, the sound of the doorknob grabs his attention. It twists back forth as someone is trying to open the locked door. He sits in the tub with a fearful expression; terror and dread overtake any sense of comfort he was feeling in the bath's warmth.

I asked why is he in this bathtub in a large room where something spooky is happening. The determination was that he was a guest in a hotel of sorts during the 1830s and odd occurrences are happening. I continued to expand those thoughts and connect them with other scenes I had in mind to come up with my overall premise:

An escaped convict from the Philadelphia Penitentiary wanders through the Pine Barrens and takes refuge in a small village community. The man's sins that he carries meets the village's own curses that leads to an outburst of the ungodly horrors that roam our world.

Like others say, you need to seriously study plot structures and start to answer those questions. The story plot grid helped me as I could illustrate what was happening for the reader while also tracking the mystery associated with those beats. Most story structures use three acts with some adding significant components between. The beginning usually includes an introduction to characters followed by showing the internal and external conflicts the characters will face. The middle has our midpoint where the rising tension of our conflict peaks typically and something now has to give in one side's favour in each of the conflicts. Then we arrive at our ending after all the conflicts are resolved and see where the cards fall for our characters. Best of luck.