A lot of people want to know this, after the success of DarkElfa's animated glove skin. A couple have been made after his, simply by experimenting on what they know, me included.
But for all those who simply need to be told how, this tutorial is being written for you.
The Tutorial
To start off, you will need to make your skin. Open the skin you wish to edit and animate. For the purposes of this tutorial, I'll be showing my glove skin.
Edit the skin to however you need as the base skin. Next, create the sections for your animation.
E.g. For my glowing tatoo, I made new layers, each with the tatoo on in the same place, but each one had a slightly larger glow on it.
Hide all the animation layers apart from the first frame.
i.e. on mine, the tatoo with no glow is the only layer visible on top of the skin.
Save this image as a TGA, at 32-bit, and call it 'spray001.TGA'. You'll find out why in a minute. Then, make the next animation layer visible, and the last one invisible.
i.e. On mine, the glowless tatoo is hidden, and the tatoo with the slightest glow is now visible. Save this as 'spray002.TGA'. Repeat this for each frame you want on your skin. I don't think there are limits, but try not to go too high, as it may cause problems loading large files in-game.
Vtex extraction - if already done, proceed past the yellow line
To get Vtex working, you will need to extract to somewhere. I find it easiest to create a new folder called 'Vtex' on the desktop, and put all the vtex files in there. Make sure every folder and file goes in there, or it will not work.
Creating the Animation
Now that Vtex is set up, it's time to get all your TGA files ready for converting. In the vtex folder, go to hl2/materialsrc/animated. You will find a txt file called 'spray'. Open it, and set the end frame to however many frames the animation has.
Place all your 'sprayXXX.TGA' files into this same folder as the txt. Make sure they all have 001-00X at the end, X being the number of the last frame.
Go back to the vtex folder and run the 'convert' MS-DOS batch file. A small dialog box will open and the TGAs will be changed into one vtf file. If an error message comes up in the box saying 'could not find file spray00X' or something similar, then you didn't set the end frame value correctly.
You will find the file in hl2/materials/animated now, along with a vmt file. Ignore the vmt, and simply copy the vtf to cstrike/materials/models/weapons/v_models/hands. If any folders are missing, then make them.
Now create a new text document with wordpad in the same place as the hands. Copy and paste this into the text file.
Make sure to set the value after "$frame" "X" to however many frames your animation has
Save this as v_hands.vmt. Now, re-name the file 'spray' to v_hands.
Go into CSS and they should now work.
Thank you to DarkElfa for letting me make this tutorial. Any questions, contact me. Any improvements needed to be made? Contact me.