I use one spread sheet for the party containing all their stats and magic items. I use this for my to hit rolls, but also to trace their silver and XP.
Per session I also do a spread sheet of the mobs. Also with their stats any special attacks legendary actions etc. For caster types I select 3 or four spells which they most likely will use. I also use this to track XP gained and money found. I also write down HP like this : 3d8+6 9-30 17. I will use 17 for that mob if it is single.
If there are four of them they will have e.g. 12, 14, 17,21 or so. When combat occurs I take another blank sheet of paper and note e.g.
I make vertical slashes to count down their HP. It helps me to track them no matter if Theatre of the mind or minis.
For the plot I do a whole reading of the purchased material, and then I note down some bullets, what is likely to happen in which order and what is not to be missed e.g. major clues NPC names etc.
Then I do a rereading to see if this matches with my take on the official stuff which for sure made me leave or add stuff.
This second reading is of utter importance, more than once I discovered logical fallacies which hi could correct but would have been embarrassing if I would not have found the m upfront.