While card set creation framework is simple and easy to use in Card Artisan, it is prudent to make some preparations in advance. Just to streamline the process a little.
This little handy check list, complete with reference to some technical nuances, will make it as easy for you, as it currently is for me.

No card set is ever made without The Idea...
In our particular case, the idea of some fictional world(s) will likely be the core of our card set.
While picking the universe is a good start, we can make few decisions right at this step, which will help us down the line.
ALOT.

Each card set have to have exactly 6 (six) factions to assign cards to.
And you, the Creator, have to decide, which characters (and effects, but more on this later) belong to which.

Default faction names are (in order of appearance in modding menu):
- Sharpshooters (white card border)
- Beast (green)
- Arcane (blue)
- Knightmare (red)
- Celestial (yellow)
- Accursed (purple)

While border color does not matter alot by itself (you can fine-tune borders in third tab of art configuration menu), modded <-> vanilla faction does matter.
Because different factions have different pools for generic low-rarity card names (and assotiated effects).
Whatever you'll use as Faction #5, it will tend to have generic names like "Cleric" or "Blessing", for example.
It is prudent to premeditate a little, which vanilla faction will serve as a base for your modded faction, because of that. So "junk" low-rarity cards won't gonna be as immersion breaking.

On top of a set of generic names and effects hardcoded for each faction, you have to assign up to 2 (two) Keywords per Faction, which will offer the access to assotiated effects (and generic card names) to these Factions.
Official keyword <-> effect checklist is here:
https://test1.login.steam.hlxgame.cc/sharedfiles/filedetails/?id=3574566077
While keyword names are quite sensible (say, Gentle does exactly what's written on a tin), it might come handy to consider, which your modded faction would gonna get which keyword.
Because one keyword can be assigned to up to 1 (one) faction - and you might consider splitting your characters between factions differently after checking keyword list out.

(and yes, there are spare keywords which SHALL be left faction-less)

And only then comes the juiciest part.

Card arts.

You need at least 10 artworks per faction, 60 total.
No less.
Duplicates are allowed.

While general-purpose 1024*1024 PNG image will serve well as a card art, you might want to crop your images in such a manner so each will have 150ish pixels of free space at the top and the bottom.
The reason is in card composition: ribbon with card name WILL overlap with card art most of the time.
And the default image size assumes that you WILL have some covered art at the bottom as well.
You can adjust art position with a special slider in 1st tab of your mod configuration.

As each and every card art can serve as both Spell and Minion, it is prudent to pick such images (and trim these in such a manner) so these will serve well as both.
I. e. it's good to have at least some character doing (or suffering) some action on each card.
It might look silly otherwise.
But it's a mere recommendation, not a blocker, even for commercially successful CCGs. You should've seen card arts on old sets of VtES CCG... It's hard to surpass - so you and your artworks will do good in comparison, that's for sure.

NB: JPG/JPEG image format is fine too.
But, since it's lossy by definition, and as you might want to CAREFULLY LOOK IT OVER from time to time (mouse scroll up while in battle to see fully sized card you're pointing at), it would be nice to pick PNG as preferred image format.
After all, you can't spell IMAGI[E]NATION without IMAGE - and it's card art which stitches everything else together into believeable game of fan favourite unofficial cards!

Also: 1024*1024 is a sweet spot at which images will look good regardless of graphics settings.
You CAN use 512*512 to save some space.
It shall look blurry on all presets except best.

Each card art is supposed to have three names assigned:
- the actual character Name, which will be used for rare cards with strong effects
- the Title, which will complement Name for some very strong rare cards - and will serve as a name substitute for more generic/common cards (minions mostly)
- and Action (nouns expected, however, few verbs here and there will do no harm), which will form a base for spells naming.

You CAN leave these blank - and the game will do its best to fill gaps with generic (and faction-generic) names.
But it will add significantly more FlAvOuR if you will add these too.

Sliders below name input control how often specific naming caregories will be used mod-wide.
Leaving these at defaults is a good move.
Too high or too low - and you'll start seeing duplicate names and other oddities.

Do note that all card art attributes are stored right in card art FILE NAME.
Yes, file name will change once you're done fiddling with it in in-game card set editor.
Plan accordingly.

It also means that you don't need any special software to edit your mod content on the go...

And, since each modded art file name starts with unique identifier, you CAN have two different card arts with completely same sets of card names assigned to these.
Which MIGHT result in cards with same name but different (or, once in a blue moon, SAME) effect and different card art being generated. Unlike crafted cards (which MUST have unique name), it's absolutely fine.
NB: generated cards with same name (and effect, even) but different arts are being counted as completely different cards for purpose of deck limits and triggers for Variant appearance.

Basically, that's it.

Six factions.
Two keywords per faction.
Ten card arts per faction.
Three names per card art (optional).

And you're ready to go!

If you'll decide to set a card back (size will be 550*750), don't forget to select it on third tab of your mod configuration.
NB: sometimes it CAN be seen upside down in enemy hand.

As of second mod configuration tab, you can commit to filling in with flavourful texts (will be stored in your mod's modinfo.mod) and character images (2048*2048) to be seen between floors and on win/loss - but it is absolutely not mandatory to do so.

And one last thing.
Path to folder from which game fetches mods is
/%PATH_TO_YOUR_STEAM_LIBRARY%/Card Artisan/Card Artisan_Data/StreamingAssets/mod
In other words, find game's folder and dive into last folder (if sorted by name alphabetically). You'll find it in an instant.

P. S.
Don't fill mod description before rightmost button will appear in Workshop Upload menu.
If only Create is active, it's too early for descriptions.
If Upload is available and pushable, that's the right time.
And don't forget that uploaded mods have Visibility: Hidden by default. . .