If you want a list of things to do to end up with sprites like what you end up with in Diablo / Diablo II / StarCraft / Fallout, then the first step is to create your 3D model, texture it, and animate it (walking, attacking, hurting, dying, etc).
Then, when all of that is done you want to set the camera to the same angle as what you would see in the game.
Then you would turn your model 45 degrees and save pictures of each stage of each animation, and keep doing that until you had pictures of every model, in every animation, from every angle.
To save space, things that didn't make sense to happen at every angle didn't.
Treasure chests were not shown from the back, et cetera.
Some games had characters that could look straight up and down, and straight left and right, but other games only used characters who were facing on 45 degree angles.
To add multiple kinds of armour, using the old-fashioned Diablo II procedure, you'd have to put each KIND of armour on your model, and save out all of the pictures in all of the poses in all of the directions.
I say "KIND" of armour, because most of the armour was just recoloured versions of the same model.
They did that recolouring in the game. So you'd make one kind of gauntlet, and add that to the model, and then in-game, you'd just recolour it.
BUT if each type of character needs to look different in chainmail, then you need to model each kind of character wearing chainmail.
There are more-modern ways of doing this... ...but if what you want is for the game to look like Diablo II, then this would be the way to accomplish it.
Saving all of this to a sprite-sheet is relatively simple:
You take all of the little pictures and you copy and paste them all into one big picture.
For the purposes of importing the sprite-sheet into the game and making it dynamically-usable, or for the purposes of using "masking" to change the colours of armour, et cetera, those are outside of the scope of your current problem.
Also, if this sounds like it's a lot of work to do manually, it really, really is.
But that's why you want to look into "Batch Processing". So that you can write a script that says "Take all of the versions of this model, and for each animation, save a picture every X % of the way through the animation, and name it
You could then write a batch process which would take all files in that folder, and put them in order, or put them out of order... ...or make one huge spritesheet, or just make one spritesheet per character, per armour type, et cetera.
A more-modern game might treat the character like a ragdoll.