Well excellent questions there!
Let’s tackle the last part first. I, for one, have been playing for so many years, its kind of second nature when to throw in encounters or monsters. You really have to know your group to use this method of building encounters. I normally like to have my game with 30% to 60% of combat, broken up during the 4 hours we game. I lean more towards role play and presenting situations of exploration, discovery and puzzles.
A friend I used to game with many years ago always said, “You don’t force us to do anything in game, you present us with the situation at hand and allow us to react, and from our reactions, you counteract.”
That’s how a DM should act during a game with their plan, always reacting to what your players decide and do.
It does take a bit of fast thinking and improv. Some people this comes naturally, others you learn to think on your feet over time.
Now onto the monsters themselves.
I normally like to pick my monsters ahead of time in my head. I know what monsters work with what level and what doesn’t from all the years of playing. If I have a question about if something is too much or too little, I pull up Kobold Fight club, plug in the numbers and check. After a while of doing this, you wont need to do anymore and can just randomly throw things together for an encounter in your head at the time.. I did a lot of checking when the game was first released to double check what works and what doesn’t work.
Far as remembering stats, I don’t stick to the book, I make my own up as I see fit. I like doing this to keep the players on their toes (yes, I tell them this info during session 0). But when I was first starting out as a DM, I would cut and paste print outs into my DM binder for quick reference and when you are a kid, playing everyday like I did back in the ’80s, you tend to memorize this (useless, lol) information.. in every edition most of the stats stayed the same, ranging a little here and there.
Other ideas I’ve seen DMs do, is keep the book at the table, and put little sticky tabs on the pages you want for the night and reference when needed.
There is various apps for the iPad/iPhone such as Fight Club 5, or Game Master (GM 5) that help you build encounters and save them for later. All you need to do is flip open your phone and boom, there is your creatures, and you can keep track of HP too.
It really depends on your style.
As always, you too can ask a question by either emailing me directly (see top of the blog for address – ASKTHEDM(@)THEEVILDM(.)COM) or clicking on the link at the top and filling out the form, which will drop an email in my box.