From: Bodhisatva Paperclip
I learned a lot about making avatars out of attachments (I make tiny ones) by *dissecting* the free ones. Once you get a grasp of basic building, do a little of that, too.
I'll second that. The best way to understand how a Furry avatar is created is by examining a full-permissions one that you can take apart, examine the scripts, etc...
In short, to do a good job creating a Furry avatar, you need to learn the following areas:
1: Basic building skills. Start at the Ivory Tower of Primitives, and learn how different shapes of prims can be assembled, using the in-world building tools that are part of the Saeond Life Client. Also devour the Building Tips forum, which has a wealth of information available for free.
2: Basic texturing skills. Start at the Livingtree sim, and Robin Sojourner has a FANTASTIC free tutorial on how textures work in SL. You will need some sort of reasonably good extrernal 2D graphics application to make textures. Photoshop is what a lot of people use. Gimp is a free but more difficult to learn/use alternative. At a minimum, the program needs to be able to save jpeg and tga (targa) files. Also devour the Texture Tips forum, which has a wealth of information available for free.
3: Basic scripting skills. Go to the LSL Wiki and learn how to write programs in LSL, the scripting language that makes things do more than just sit there as static objects in SL. It's a fairly straight forward programming language. You use scripts to do things like make tails wag and eyes blink and ears twitch, and to change eye colors and fur colors and smiles and mouth positions, all by commands, or on-screen controls, or reactions to various conditions.
4: (Optional, but the good avatars use this) Learn to make Sculpted Prims. This takes an external 3D application like Wings3D or Blender. Start in the Building Tips forum, for tutorials and advice on software to use. Sculpted prims can give you shapes that are more organic and less harshly mechanical than normal prims.
5: (optional, depends on result needed) Learn to make animations that shape and deform an avatar's body. There are some forms, like a 4-legged normal-sized wolf, or a 10 foot tall ogre, or a 1 foot tall squirrel, or a small butterfly, that simply can not be done without warping the normal avatar body out of shape, and overriding the normal movement animations. This is advanced stuff, but makes some of the avatars possible that don't look like just a Human wearing a mask and tacked on tail and shaped legs.