So first things first, you have to make the texture that you want in your map. So, open up your image program, and create it and save it as a dds, jpg, bmp, tga, gif or png, named whatever you like (I’ll be calling it CustomTex01). So you should now have a picture called CustomTex01 saved somewhere on your computer

Once you have that, you put it into your "Steam/steamapps/accountname/counter strike source/cstrike/materials" folder (you can make you own folder to put it in) I will be putting it in a folder at this location
Steam/steamapps/accountname/counter strike source/cstrike/materials/CustomTex".

After you've put it into your materials folder, open up VTFEdit, and import your image into it. Then, select File -> Save As and name it what you'd like(again, I’m going with CustomTex01). After you've saved it as a VTF, you select Tools -> Create VMT from the toolbar of VTFEdit.
It will then open up a second, smaller window, which will have several options available to you that allow you to give your textures things like bumpmaps, or normal maps, however we won't be doing any of those in this tut. Now, at the top of the second window there will be two tabs, one called Textures (the first one you see) and one called Options. Select Options and now you're able to choose the properties of you texture, such as what kind of material it is (metal, glass, wood...). Select whatever you want as Surface 1 and you can also set a second Surface (but it's not necessary).

Now, what you have to do is start up Source SDK (if it’s your first time using it, and it doesn’t load up [it says it’s starting, but just goes away] put this line of code in its launch options: -applaunch 211 -beta sdk -engine ep1). Now, after it starts, open up Hammer World Editor, and open the map you are trying to place the texture into. Now, select the brush or whatever you are trying to apply the texture to, and select the Texture Application Tool, and Filter to the name of your texture. If done correctly you should see your texture. If the VMT was incorrectly set up however (due to a wrong location specified in the VMT), then you will get a purple and black checkered image. Place the texture wherever you wish in the map.

After you’ve placed the texture into the map, you can compile it. But you’re not done once it’s compiled. Now you have to add the files to the bsp. We do this by using PakRat.

So, after you’ve downloaded, setup, and installed PakRat, you open it up. Now, you will be presented with an Explorer like window, asking you to search for your maps bsp file. Once you’ve selected it, it should change to show a different window, with the name of one (or more) textures. You will notice that yours is not in that list. So, you must add them to the map. To do so, click on the Auto button at the bottom of the screen. Now another window opens. In this one, you press Scan, at the top. It will then show a list of the textures in the map. You must then select all of the textures you want to be included within the map (which means selecting the VTF’s and VMT’s). Then, you select add to map, and finally you click Save

Congratulations, it was a long process (and I’m sure reading all this didn’t help the boredom), but if you do it well, it will have been a worthwhile process in separating your map from the rest.