I have read through (and still read through) the DM Guide section of Creating a Monster; so I am familiar with their guidance.  For this thread, I want to get some first hand input from people who go through the process of creating monsters based on external source materials.  I would like to hear some "lessons learned" discussion, tips and tricks for putting a monster together, or just advice on what to avoid doing. 

I spending most of my nights this month watching a continuous stream of horror movies and I have been inspired to dabble in creating a monster character sheet based on some of the creatures I see in the films.  It is fun trying to translate some of the gimmicks from these iconic monsters to new features and attributes that would work as challenge for high level characters in the game.  I am not that proficient in calculating a CR level, so I am taking the approach of building out the character and then going back to see what the rating would look like. 

Anyway, I tend to try to base the monsters (all are currently a humanoid type) to one of the PC playable classes.  I then strip away the class attributes that are more player focus or restructure these features in a way to make them unique to the monster.  I find that starting this way helps with determining things like the Hit Die, bonuses on attacks, and even a breakdown of spells.  Again, I don't straight up build it like a PC and copy the class completely.  But a Barbarian build inspires me to plan for "natural armor" and "damage bonus" while a Fighter build makes me think about 'attack variety" and "defensive tactics".  I am not sure this is the best way to approach it, but it works for me to start somewhere. 

Regardless, I can't help but be inspired by the characters I am watching and it is just too darn fun to try to make a D&D variant of them.  So what advice or stories do you have?  Do you ever get inspired by the BBEG in movie, TV series, novel, and feel compelled to create a variant and try to run it a the table? 

Thanks for your thoughts and hope you are having a wonderful night. 

Now, there is nothing wrong with using a PC sheet for your monsters or your NPCs. Nothing at all.

The CR system, however, does not work, and was never meant to work or be used with, a PC style character sheet, in any way.

So, one of the first things to realize is that you don't use character sheets for NPCs or monsters.

You use stat blocks.

Monsters are not limited by classes. You can have a Boogeyman who takes features from druids, fighters, and rogues if you want. Better yet, a Boogeyman probably doesn't need any of those features.

To figure out the CR for a given monster, look at how it is portrayed --indeed, you already have your way of figuring out their ability scores. 

Next, what's the Proficiency bonus they have? What armor class would you assign them?

How big are they? Big matters because that determines the size of the hit die. Most of the kind you are probably thinking about will be d8 or d10. How many will they have -- how much abuse can this monster take? that will give you an idea of the hit points.

Attack bonus is easy. Damage per round is a little more involved...

DPR is the average of each attack added together. So if a creature does 1d6 damage with each attack and has 5 attacks, the DPR for this is going to be 3.5 (the average of a d6) times 5, or 21.

Lastly, the Save DC is basically how hard it is to resist whatever they have thrown at someone.

That gives you the different numbers you need to look at the chart in the DMG and see what their CR is going to be. Us the row with the highest number of closest matches.

that's basically the DMG way and CR set up.  The rest of ti just talks about tweaking and adjusting.

I use a homebrew CR set up that allows me to flex and shift any of 25 different features, because i needed to have a more flexible system since I had to create all my creatures and critters, monsters and madnesses. I had to use old legends and myths, folklore and more, and then also I use monsters from movies (1980 or later) and novels (anything written after 1985 or before 1920, specifically).

I have 125 monsters, several of which have variants and that "get stronger" over time/level up.

None of them use a character sheet. I don't even use character sheets for important NPCs. Everything is a stat block. Stat blocks are, basically, the DM's Character Sheet. Those are what the CR was meant to be used with.