How do i use a ready VTF weapon skin in CS:GO

I answered a similar question to this: How Do I Get More Guns?. Noticed that your question was posted a few hours before that though.

This is the new "Arms Deal" update which went live on 14/08/2013. Valve released a really good FAQ on it, which can be found here.

The bits of interest for your particular question, "How do I get item drops?":

• Timed weapon drops, just by playing on official and community servers (VAC enabled) in any game mode.
• Opening dropped weapon cases with a weapon case key.
• Trade weapons with other players through Steam's Trading interface.
• Purchase them from other players through the Steam Marketplace.

Regarding the first point, these are obtained for play time over the course of a week. As your play time increases, the drop rate will slow down. This is reset when the week rolls over. Not sure the start day or time of a week though.

I'm not 100% sure where you will get the weapon case keys. The below quote suggests that you buy these:

This update will also actively support the CS:GO competitive community with the introduction of the eSports Weapon Case. A portion of the proceeds from sales of eSports Case Keys will go towards prize purses at competitive events.

Hope that helps.

Counter-Strike: Global Offensive's workshop works differently than most Steam workshops. You cannot simply browse to a file and add it to your game. Instead, the workshop is used to see items that maybe added to the game as drops or a part of a weapon case, or ones that have already been added.

Once you acquire the weapon (that particular one is already in the game and can be found by opening a winter offensive weapon case), you can use it by going to your inventory in-game and replacing your current weapon with the new one by right-clicking on it and choosing "Replace T", "Replace CT" or "Replace both".