Make a chart.
J.K. Rowling did.
Honestly, I kind of do this, and I kind of don’t.
I give each plot thread its own blank page and then start writing down every major twist and turn and cup of tea I want to cover in that particular thread before the book ends. Then, I take my collective pages and start organizing all of that activity into chapters. That organizational effort sometimes makes its way into a chart, but, if I’m being honest, I usually don’t stick to the chart in the end. I’m a booger for continuity and timing and flow in a story, so I’ll bend and break that chart until the narrative glides along like a rolling stream.
For this reason, I sometimes forgo the chart and just make a river of notes in my manuscript.
Do whatever works for you, but definitely put in the work of organizing your content. You’ll write much faster that way.