An important lesson you learn when studying figure drawing is to work from the biggest shapes down to the smallest, starting with the general shape of the head before moving on to the features of the face within it.
In Koikatsu this means starting with the head scale X, Y, and Z sliders, then continuing with the ears, jaw and cheeks sliders, I know that the sheer amount of sliders can be daunting but keep in mind that you can always go back and fine tune them, at this point of the process you're not supposed to be concerned with the details of the face, we're trying to get a satisfying head shape to work on as a foundation, think of it as sculpting a head with clay, you don't start with the smaller features of the face like the eyes or the nose.
Since your references are also 3d models this part shouldn't be that complicated, just make sure to angle the head of your model to match the same angles from your references and switch between them from time to time to get as close as possible. If you have a card that has a similar shape of the head you can copy the slider values from it to save yourself some time and work from there, the card doesn't have to be exactly the same as the one you are trying to match, it could even be from a completely different character as long as it has a head shape that you feel can help you get closer to the final result, otherwise you can make the head from scratch by observation, just take your time and work on the sliders one by one.
Once you're satisfied with the head shape you should start laying out the face by roughly measuring where each feature sits in the model you're trying to match, the sliders I move at this point are eye, eyebrow, nose, and mouth vertical position, as well as eye height, width, and spacing, don't concern yourself with the shape or details of the features just yet, we are laying out where each feature sits, not their specific form. Having basic anatomical knowledge helps; the eyes sit roughly at the middle point between the top of the head and the tip of the chin and are about one eye apart in width, the base of the nose sits at the middle point from the tip of the chin and the center line of the eyes, etc. I'm aware that waifus do not look like real humans most of the time but in my opinion these are still good guidelines if you find yourself having trouble placing the features of the face, and if you look at my Chun-Li card and P.Jackson's Mai these rules still apply despite how stylistically different they are from real life.
From Andrew Loomis' Drawing the Head and Hands
Lastly we concern ourselves with the features of the face as well as other character specific details, I usually start with the easier ones like mouth width and angle, eyebrow shape and spacing, and leave the eyes for last since they tend be the trickiest to get right, first with the eye rotation slider to get a basic idea of the shape, then the eye lid sliders, inner and outer eye corner height sliders and finally the upper and lower eyelash sliders, I personally don't change the eyeball scale sliders at all since I find the previous options robust enough to get the desired shape but feel free to mess around with them if you wish, additionally don't forget to play around with the iris shape and vertical position sliders as well, since a slight adjustment of the iris can make or break the appearance of the eye from my experience. Regarding character specific details, these are the unique facial features of the character you're trying to match, for example, going back to my Chun-Li card, it was important to get the eyebrows and red makeup right since these are iconic details of the character to me, this just comes down to looking at the list of face feature types in your game and find the ones that look the closest to your reference (or you could draw them in photoshop or any other raster graphics software but this guide is already long enough so I won't cover material editor and overlay stuff here lol). Ask yourself what are the details of the card you're trying to match that stand out the most, it could be the makeup, eye color, eyebrow shape, a mole, etc.
Chun-Li 3rd Strike portrait on the left, my card on the right
To summarize, start with the largest shapes first (the shape of the head, jaw, and chin),then lay out where each feature of the face sits inside the head by measuring your reference (eye, mouth, and nose vertical position and spacing),lastly work on the smallest details like eye and iris shapes and don't forget to get the character defining features like makeup or eye color. If you give yourself some structure in your work process you won't get overwhelmed by the vast sea of sliders that the character maker has.
Some closing thoughts
From personal experience it's very easy to get tunnel vision and get frustrated from not getting a specific detail right, I've read of artists dealing with this by looking at their work through a mirror, flipping their canvas upside down or looking at their work from a really far away distance, I find these pretty useful when I'm stuck but if they don't work for you just take a small break and you'll be just fine.
I also recommend saving your character multiple times as you work on it because if you feel like you messed something up and don't remember which sliders you moved you can always go back to a previous version of your character and work from there, for example I saved the Mekami Shifty card I posted like 7 times, each with a slightly different face, because I couldn't decide which one to keep working on, and even after I posted it I made another version lol, make as many copies as you can and don't be afraid to mess around with them.
I hope this guide helps you or anyone else trying to make character cards, thanks for reading till the end.
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