Step 1: Pick your hair

Not every hairstyle will be a good fit! Since Sims and Paras don't have the same head shape, any hairstyle that shows the hairline or head will require a lot more work to shape it and remake the textures. For your first try I'd recommend a medium-length style with no hairline showing. I'm using Sienna's hair from Horse Ranch, which only has a bit of hairline on the right side of the head, to demonstrate.

Once you've picked the style, open Sims 4 Studio. Select "Create CAS Standalone" and hit "CAS." Select the grey or greyish white variant of the hairstyle you like, and create a dummy package (you can delete this later).

Now, under the Texture tab, make sure "Diffuse" is selected and export the texture. Name it something or place it somewhere so that you'll remember it's the original. Go to the Meshes tab and export the mesh.

Step 2: Prep The Hair

Open up your original hair mesh in Blender. It'll come with a bunch of crap you don't need, plus three files labelled s4studio_mesh_1-3. That's your hair mesh and the two hat chops. Shift+select those three files and export as FBX. Make sure you tick to limit to Selected Objects. Now you can open up HumanRig and import your hair FBX. Sims are bigger than Paras, so you can grab all three of them and scale them down in layout mode, I chose 0.85 in this project. There will be a weird cap attached to the hair- move the whole object so that it roughly lines up with the head (it doesn't have to be perfect!), paying careful attention to matching the ear position. This cap will be your hairline later. Make sure all your hat chops are all still aligned.

Once all of this is in place, apply all transforms and reset your origin to the 3D cursor. Do this any time you need to adjust it in layout mode!

Step 3: Remodel VERY CAREFULLY

Make sure all three meshes are still selected, and switch to edit mode. Any changes we make to one mesh should also be made to the other two. When you need to move a vertex, switch to X-ray mode and drag to select all overlapping vertices. Enable proportional editing to move larger areas smoothly. Since paras don't have the same head shape as sims, the skull cap won't match up perfectly- we want to approximate a smooth transition by burying the edge vertices in the head, but keep the rest visible so that the hairline shows up correctly. Be careful not to rip any vertices or create any differences between the three meshes. If you make a mistake, don't be afraid to restart. You should also fix any other issues at this stage- sims have huge foreheads, so my para's head was sticking straight out the top of this mesh, I used proportional editing to carefully lift it up.

Step 4: Fix your hat chops!

Hat chops are the alternate version of the hair that is shown when the character wears a hat. In both Paralives and TS4, there are straight and angled hat chops. Identify and label which is which. Under the HatStandards folder, there are a number of reference models we can use to check our work. For each chop, carefully move the vertices with proportional editing so that they match up with, but don't exceed, the "hair limit." With the angled chop, turn on the "HatStandardAngled" model to test if your hair is visible through it- if so, bring it closer to the head.

Step 5: Oh, Textures...

Textures are maybe the most annoying part of this. Load in your base texture as the Base Colour for the material your hair is using. Looks nice, right? Probably not very Paralives-y. Paralives uses a much flatter and more cartoony rendering style than TS4. To fix this, we're going to edit the texture. Now, regardless of what happens, you're going to have to edit the UVs at some point- I'll be doing it at the end and showing you how, but if you find it easier, skip to that step and go back here when you're done.

Here's a sample hair texture from the game and the hex codes it uses. We'll want to adjust the brightness of our texture to roughly match, that way it won't show up too bright or dark in game.

I have a clever strategy for this in Clip Studio. I make a rainbow gradient map, and adjust the colours until they match each zone.

Now that we're in a good colour range, I'm going to separate any important hairline details onto their own layer, lock transparency of the main "block," and start messing with it until it's a good base to draw on. I posterized it with 11 colour levels and then did a gaussian blur for a decent base. Then I went in with a brush to exaggerate and smooth out the shapes. The Paralives team uses a very blendy gradient with some fingertip pulls to make it look paintery, then some more defined shadows. I go in with my dark pencil to sharpen up the shadows and then blend out the other colours.

At this point, you can start saving your file and importing it to Blender. Check what it looks like and start making adjustments. If you find an issue, you can bring the model into Texture Paint mode, paint a dot on the offending piece, and see where the dot appeared on the map to quickly reference where the problem is.

The rest of the painting process is just art! Try replicating a texture from the game that's similar to your hairstyle.

Now that we're done with the fun part.... it's time for UV hell.

Step 6: Suffer

I've never met a single artist who enjoys doing UVs. Luckily, we don't have too much to do here.

Open up the UV editor and replace the background image with the Parafolks UV Layout we downloaded earlier. Now, switch to edit mode and grab all the vertices of your hair. Under the UV tab, select "Export UV Layout." Save the image using a name that indicates it's the original. Now grab that big box of vertices and move it to the right and down so that it lines up with the cyan box labelled "hair." Move the skullcap part up and align the ears. If your hair has extra textures hidden within the skull part of the head, you'll need to move these around to fit into the remaining space in the purple "head" zone. Once your UVs are finished, export them again, this time marking them as the new ones.

Okay, you survived that far. Now back to the image editing. Import both the original and new UVs to your texture file. Place the new UVs below your texture layer, the original UVs over it, and set the texture to a low opacity. With both the original UVs and texture selected, draw a selection box around your square of UVs, and move it to the bottom corner. Place it so that the UV lines match up exactly, pixel for pixel. Voila, now your texture matches the new UVs.

There's just the issue of the hairline (and, if you have them, any pieces hidden on the skull section). Sadly, there's no way around it, you're gonna have to do it manually. Grab each piece using the same method and the lasso tool, and line them up one by one. For hairlines that you had to manipulate, align as much as you can with the least stretched part of the uvs, and then cut, rotate, and stretch them into place. This is why I didn't paint the hairline during the last part. Now that it's in place, I paint it to match.

Step 7: Set up the mod!

Now that you're happy with your hair's model and textures, you can follow this official tutorial to skin it, create a mask, and set up the mod in-game. This Sienna haircut is not available on the workshop yet because uh.... well it's definitely not because I got lazy and didn't feel like skinning it today (the only thing worse than UVs is weight painting).