Skin Tutorial
A step-by-step tutorial by Sean Baity
1. What you need to make skins for The Sims
You will need the latest version of SimShow, the Sims model and texture viewer.
It can be downloaded from http://www.thesims.com
You will also need an image editor such as PhotoShop or Paintshop Pro.
There are many evaluation and freeware versions of editing programs available on the Internet.
2. Understanding Texture file names.
Each texture name includes a series of letters and numbers such as "B003MAFitlgt" or "C003MAlgt". These are necessary because those letters are a key as to which mesh or model the texture is to be applied to. Listed below is the key breakdown.
B= "body". Body textures are separate from head textures so the body texture must begin with a "B", as in "B003MAFitlgt_Dad02.bmp"
C= "cranium" or head. Head textures are separate from body textures so the body texture must begin with a "C", as in "C003MAlgt_Dad02.bmp"
The three numbers after the B or C refer to the type of model.
For Bodies:
001 = short sleeved and shorts.
002 = short sleeved and long pants.
003= long sleeved and long pants.
004= coat and pants formal attire.
005 = short sleeved dress for women, regular jacket and tie for men.
M = "male". Male models are obviously differently shaped than female models.
F = "female". Female bodies are different from male bodies.
A = "adult". Use this key for adult models.
UC = "Universal Child". Use this key for child models.
Fit = "Fit". Just like it sounds this is for the fit models for male and female.
Fat = "Fat". This key is used for the fat models for male and female.
Chd = "Child". All children have the same size bodies, which is fit.
drk = "Dark". Use for dark skin tones.
med = "Medium". Use for medium skin tones.
Lgt = "Light". Use for light skin tones.

For heads:
Please check SimShow viewer, as they are difficult to describe. The heads differ by hair, bandanas, glasses. Etc…
After all those keys there will be an underscore "_" and then either numbers or names. This is where you would customize your skin by giving it your own name. You must preserve the underscore character "_".
So if I like the way B003MAFitlgt_Dad02.bmp looks in SimShow, I can make a new skin and rename it B003MAFitlgt_MAXISean.bmp and it will automatically fit the same model.
What you will learn.
I am going to show you how to turn all these:
 

into these:

3. Editing textures
First step-find what you want to change:

The first thing you want to do is identify the model you want to make a new skin for. Open SimShow to see what is available. For this tutorial, I have decided to use a long sleeved and long pants model. I can tell what textures I will need to modify by checking the model info area of SimShow. It looks like I need B003MAFitlgt_Dad02 for the body and C003Malgt_Dad02 for the head.
 
Step 2-Finding the texture:

Next you need to find the actual textures you want to edit. All the textures are stored in the SimShow\GameData\Textures directory.
Since you don't want to overwrite the existing textures that came with SimShow, I recommend copying them to another temporary folder before you work on them.
Just make one called c:\temp texture or something like that.
Step 3-Editing the texture:
So here are the textures I want to change.
As you can see the areas where the fabric and skin are in the texture are stretched all the way out to the edges. This is necessary to cover every portion of the 3D model with a texture. Since I will need to paste my own images into or over these, it will be necessary to edit them in RGB color mode or 16-bit color.
Convert the existing 256 color textures to RGB or 16-bit color mode.
Next I need to paste my own images into the existing ones to customize them. Here's my lovely mug shot. Don't I look happy?
That was before I worked at MAXIS.
Since the shirt can be done on the body texture, I need to only cut the portion of the picture absolutely necessary for it to convey my image.
Here's what I need. Next paste that into the existing face. Oops! That doesn't look exactly right. First I need to stretch the pasted image to about the same proportions as the face underneath it so the position of the eyes and nose and mouth match. In PhotoShop, I use the "free transform" function to stretch the entire rectangle at once.
I usually pull the new face layer off to the side to compare nose and eye sizes. The head model does have a nose so that needs to match pretty well.
Well now the face is the right general size but whoa! Is that skin color way off! There are several techniques one can use here. I could try sampling the color from the existing face and carefully paint my face with an airbrush or paint tool. Or I could blend the top face into the bottom one by making it semi opaque, or I could change the face color to be more like the new one. I'm going to try a blending and cloning technique. First I obliterate the bottom face with an airbrush using the existing colors. .
Then I paste in the resized custom face. I then adjust the opacity just enough to let the bottom color show through but leave the
definition of the face intact. Then I carefully use a combination of the clone tool and the finger tool to blend the two. I take the clone and finger smudge tools and set the opacity down to about 20 percent and blend the edges into each other like so. But, the skin tones are still seriously off. I am going to use the existing skin tone and use that to paint over the lighter one. But I lost some definition in the face.
I'll use a pencil tool to put some detail back in. . Oops forgot to make the hat. Since I have red hair, I'll just keep the hair, especially since making it wrap around correctly is pretty difficult. I'm going to just clone or copy or paint the color of the hat around the head and highlight the top like so.
So the head is pretty much done at this point EXCEPT for the very important renaming.
Step 4 - Renaming the texture:

Since I want my new head to fit on the same cranium model as the texture I am modifying, I need to use the same model key in the name.
The name of the existing texture is " C003MAlgt_Dad02.bmp". I want my head texture to have the name "MAXISean" on it somewhere, so I need to rename it " C003Malgt_MAXISean". Now I copy this new head into the SimShow\GameData\Textures directory.
For the body texture I will show you how I edit the images as you now know the editing steps.
 
 

*Note. To get images to appear correctly on the front of your character, it must be placed in the texture backwards. Images on the back will appear normally when placed normally. This has to do with how the images are read in the game.
Step 5 -Checking your work in SimShow:

So it appears I am done with a head texture and a body texture.
I renamed my body texture to B003MAFitlgt_MAXISean.bmp so it will map to the same model that the original B003MAFitlgt_Dad02.bmp texture did.
* YOU MUST SAVE BOTH FILES IN 256 COLORS. * All the work done above was done in 16-bit color mode. Most blending and airbrush tools won't work in 256-color mode. That is why you convert the images to 16 bit (RGB) color to start with. When I convert them to 256, a little detail may be lost, but it is negligible:
 
Now the files must be places in the SimShow\GameData\Textures directory to work in Sim Show.
*Note: In the final version of The Sims, players need to drop their custom textures from SimShow into the C:\Program Files\Maxis\The Sims\GameData\Textures folder to see them in the game.

Then start SimShow. The default model on view is B001FAFat.
  I need to change it to B003MAFit in order to see my model and choose my head texture.

Here's the first pass:
Not bad but the nose and eyes may need to be moved up a notch and the seam between the front and back of the jeans needs to be reworked.
Here's the 2nd pass:
I adjusted the pants seam a little bit, but even with the facial adjustment, the texture still doesn't fit right on model C003Malgt.
I'm going to rename my head texture to C004Malgt_MAXISean to see how it fits on the biker head model.
Ahhh. Now that looks a little more like me. So I just needed to use a different model and to do that I just renamed the first "model key" part of the head.
The finished product: .(Just need to adjust my hand color.)
This could use some finishing touches obviously, but is a good place to start at least.

Step 6 - Exhibit:
Once the textures are done, you can email them to other people with SimShow or post them on your website for download. The textures you make will work with the shipping version of The Sims. Enjoy!