Unlike most things I design, I don’t simply start writing when I’m designing a monster. There’s just enough math involved for me to want some virtual assistance, so I use DND Monster Maker by The Genius Inc. This tool allows you to input the various statistics of a monster and it will automatically calculate how the stats affect other stats like adjusting the AC based on Dexterity, attack damage based on Str or Dex, skill modifiers based on the parent ability, etc. You can still manually adjust these stats if there’s something other than just the ability scores in play, but it’s very helpful to not have to manually calculate every single little thing.
Below I've included a screenshot of the program without any information entered into it and the program with the stats of a monster I designed ages ago.
When I’m done designing a monster on Monster Maker, I could export the statblock along with its formatting, though I usually prefer to manually recreate it in either Word or The Homebrewery. The Homebrewery is an online tool I use in all kinds of D&D products to format everything into looking like an official D&D product. I have been increasingly using Word to manually format my products so they stand out, but whenever you want to make something look authentic, then the Homebrewery is a great tool. There’s a small learning curve as far as what very basic coding you have to input to produce what effect, but it’s very easy to learn and there’s plenty of pre-formatted examples that can let you see how something works and work backward from there.
These are the main tools I use when I create a monster.