Session Preparation
Striking a balance between railroading and under-preparing is difficult. The notes I take into a session may look very minimal to you but the main work I do as a DM is before the first session.
I try to get everything sketched out for the plot points, settlements and NPCs ahead of time and fill a binder with them. Once the binder is done I find it a lot easier to improvise as I know history and the present of the world I'm in; to me the future is the collaborative part of the narrative between the players, the dice and myself.
So my notes really won't make much sense to you but they are a rough guide as to how the session will play out if my players don't do something weird and left field. If they do something different that's fine and I have the prep work done to improvise another course of action. Acceptance that there will be a lot of things that will never be used is a bitter lesson to learn but you get used to it, if you can't get over that then write a novel!
So the plot points are done now you move on to the NPCs, I have a section in my folder just for NPCs. Each entry features where they are, their profession, any accent I gave them, their appearance and a short history in bullet points. I will write in short notes about important interactions with the PCs and things that are on order from vendors etc.
Make sure you have all the handouts and maps ready for the places you will be going so you won't have to waste time.
Encounter preparation can be quite complicated and I will return to this next week but decide on the monsters and write down the page number and manual you can find their statistics in for reference. If you use minis then the relevant miniatures and a battlemap.
Dice, snacks, a pencil and notepad then you are ready!
You really don't need to overthink individual sessions just use your notes from the previous sessions to modify the overall story arc.
Have fun and may you roll well!