This guide is heavily based off of the DWAvatar documentation found here:
https://web.archive.org/web/20040806133103fw_/http://www.georgios.pwp.blueyonder.co.uk/worlds/dwavatar/help/avatarreference.html

I'm posting it here since the Wayback Machine can be slow or unreliable at times and I want others to more easily discover this information.

Needless to say, most of the guides on this subject have been lost to time and are only partially archived (to my knowledge). So if you have further information to add or perhaps an intact source elsewhere, I'd be happy to mention it here.

THE BASICS
Avatar codes work like a sequential script. WorldsPlayer constructs the avatar with individual parts from the associated .BOD files in order by following the instructions in the avatar code. Think of it like a LEGO manual but in key+value form.

This is an example of what a basic avatar code should look like, color coded for your convenience. Green selects body parts, red is commands, and blue is parameters. I will explain what this code does step-by-step in a moment:
avatar:tina.0EPGogreT3kbyC_000C_RC_RC_BC_000NGalexHT9kbyM0GnightglyderU0GnightglyderVC_BC_RD2SHHn.rwg

So how complex can these avatars get? Well... Pretty complex. Maybe even a little crazy. This knight avatar below has two torsos, wings, flashing props, several material & model swaps, and even Jack Sparrow's belt:
avatar:tina.0EPGaxelT3kbyT3kbyC_RC_BC_YLaGinfinitySLLLMaOaGogreSnnnRaGinfinitySLLLUaVaGogreC_RSnnn0defWaGleeXa4aSffnYaIaGleeJa4aSffnKaBbGogreSfdf4aGaxelweT3kbySHHHPSNNNaGjak0a1cNS000QGrooSHHHHT1kangaroo.rwg


Another important thing to understand is that avatars are split into different body parts in a hierarchical order, starting from the hips or pelvis. Swapping body parts, textures, or resizing can and often will affect the parts that are below the part you are modifying.

By default, all body parts and animations will be set to the base avatar you use at the start of your avatar code, for example, "achoo.bod". Replacing the pelvis will replace the entire avatar's model, but the avatar will always use the animation of the base avatar you started with. You can use this to apply any action set to any avatar you want. Using "vwbug" as the base avatar will disable animations entirely since cars do not have animations..

The structural makeup of an avatar is as follows:


Note that the Heel and Toe parts are only separated in modelling software, the WorldsPlayer does not allow you to swap these parts out and considers them both part of the same "foot" or "shoe" part. Additionally, several invisible parts without models also exist. These include all of the joints such as the shoulders, knees, elbows, wrists, ankles, base of the neck, and top of the neck. The back of the head is also it's own separate model, including the ears on animals such as the Kangaroo. This is only visible when viewing the generated PosableShape in Shaper or exporting it to modelling software, you will not be changing these when creating your avatar.

PARTS
The available body part selectors are as follows. Listed are the joints connecting the parts + the parts themselves, in that order:
Z - Tail
P - Pelvis
B - Torso
N - Neck
H - Head
W - Left Hip + Thigh
X - Left Knee + Calf
Y - Left Ankle + Foot
I - Right Hip + Thigh
J - Right Knee + Calf
K - Right Ankle + Foot
L - Left Shoulder + Arm
M - Left Elbow + Forearm
O - Left Wrist + Hand
R - Right Shoulder + Arm
U - Right Elbow + Forearm
V - Right Wrist + Hand

Normally when replacing parts, you do it to the part itself (calf, foot, forearm, etc.), however the letters actually point to those joints. When we swap body parts, we're actually swapping the parent joint. Because of this, you can also append objects to the joints (knees, elbows, wrists) to add things like armor pads.

The avatar code Ja4aG4taur adds a Minotaur head to the existing left knee, because here "J" refers to the knee bone rather than the calf of the leg itself.


WALKTHROUGH
Let's walk through that avatar code I showed you earlier:

avatar:tina
This is the base avatar. It starts with the "avatar:" prefix before pointing to one of the .bod or .rwg files for an avatar template created by Worlds Inc. You can find a list of base avatars to choose from by opening VIP > Choose Avatar and then looking through the list of Articulated avatars.

.0E
This tells WorldsPlayer that you want to modify the base avatar.

PGogre
This replaces(G) the Pelvis(P) with the pelvis part from ogre.bod, thereby replacing the entire model. We now have an Ogre using Tina's animations. Let's continue.

T3kby
This adds a texture(T) from the file "kby.mov" in your WorldsPlayer's /avatars/ folder, and uses the 3rd frame from the file, which is this image right here:

The last body part we referenced is the pelvis, so this texture and all textures after it will be applied to the entire avatar's set of base textures (usually skin, shirt, coat, dress, pants, and hair, in that order). Normally, when applying textures, the first texture we reference will be applied as the default for the body part placed before it and all of it's child parts. You can see this take effect on the pelvis model as our kbv.mov texture appears on it:


C_000
This is a solid color we've added to the second texture slot. You can see that most of the other parts have adopted it as their default material. Allegedly this is supposed to be RGB, but I've found that to not be the case. I'm not entirely sure what color format it is in.

C_R
C_B
C_000
These first two grab the preset colors (R)ed and (B)lue. The third one is the same as before where we created the color black with numbers. You'll notice all of these start with "C_". C_ is the command to attach a color as a texture. Using letters gets you preset colors while using 3 numbers gets you a custom color. Unfortunately I do not have a list of preset colors yet.

NGalex
This replaces(G) the neck(N) with the one from the Alex model.

HT9kby
This changes the texture(T) on the head(H) with the 9th frame from kby.mov, giving our character a Santa Claus face.

M0Gnightglyder
U0Gnightglyder
This is a little different. We've used the "G" command to replace a body part, but this time, we're actually not replacing it. The G command sees a "0" placed before itself, and goes back to see what body part is referenced. We referenced the M and U body parts, so it grabs those parts from the next model and appends them to our avatar. In this case, M and U are the wings belonging to the Nightglyder. We have attached bat wings to our Ogre torso.

VC_BC_R
D2
Let's use those C_ colors again, but let's try something cool with them this time. We're going to select the right hand(V) and then add the colors Blue and Red. This technique is used to create flashing parts, or blinking animations on faces. Sometimes this is done automatically depending on the texture used and how it's applied. Now, we're going to change the blinking interval(D) to 2.


SHHn
Now let's get extra silly and squish the Ogre hand so that it's fat and short, thereby also making the bow smaller. We use the Size command (S) and then enter an X, Y, and Z coordinate. These aren't decimal numbers but rather a scale from A to Z. Capital letters make it bigger, lowercase makes it smaller, and 0 makes no changes. The size command affects children, so applying the size modifier to your pelvis will resize the entire avatar. They are stackable as well. You can put several size commands in a row to get a ludicrous size, just please don't do this in public because it will annoy others.


As you can see, this is our fully constructed avatar. It looks pretty shitty, but this is just a demonstration so it's okay.


Mix and match whatever parts and modifiers you want. Just be sure not to exceed the size limit, which is about 220 characters. Yes, there is a limit! If you enter an avatar code that is "too complex", then it will be reset back to default. MAKE BACKUPS!!