Download decal convetor from http://www.fpsbanana.com or
http://www.cstrike-planet.com/files/28 or u can search it on google...![]

jJust download and install it ...and follow the following ...:

Decal Creation Description
----------------------------

01) Create a 256 color bmp picture.
02) Width and Height Dimensions must be multiple of 16 pixels.
03) Multiplication Width*Height (in pixels) must be less (and strictly less !!!) than 12288.
04) Last palette color must be pure blue (#0000FF).
05) All color pixels corresponding to pure blue will be transparent.
06) Save your picture in the 'MakeDecal' folder of the application.
07) Be sure that there no more than one bmp file in this folder.
08) Execute 'MakeWad.bat', located in the 'MakeDecal' folder of the application.
This will generate a 'PLDECAL.WAD' file in the same folder.
09) Copy this generated file into your Half-Life (or Counter-Strike, or another HL Mod) folder.
10) Remember, don't go back to the 'Customize' menu of Half-Life (or the HL Mod) after this.
11) Otherwise it will be overwritten by the default HL decal.
12) If this occurs, just repeat the 08 step seen earlier (If your bmp is still in the folder !!!)
13) Image Size should not exceed 15ko, otherwise display mistakes may occur.

NjoY ...!

To view a demo video to do it properly is as follows..!