1. Requirements to startFirst, you will need the PC version of the game, then you will also need the SA2 Tools for the editing. And the image editing tool? You will chose.
2. Texture EditorOn SA2 Tools' many folders, you will have to enter on the "TextureEditor" one. Then launch the .exe file with the same name as the folder.
3. Choose the file you want to editIn the "File" section, go the "Open" option and next go the "resource" folder in the SA2 folder (which is in Steam's files) and then enter the "gd_PC" folder. Now you can select any file you want. Check MainMemory's comment below and click the link. With that, we will know which texture you want edit.
4. EditingHere's the fun part, you will have to select the texture to edit and then export it with the "Export" option. Save the .png texture in where or make a folder for it (i recommend making a folder for the texture). Now use any image editing tool and do the idea of your mod.
5. Preparing our modNow with your texture done, import the file you edited with the "Import" option. Now go back to the "File" section and save it as "Save As". Now you need the SA2 Modloader for this. Go to "mod" folder, and give the mod a folder. Create a new "gd_PC" folder and inside of it create other called "PRS", and save it right there. It will a .pak file, but you don't need to worry about it. Create a .ini file on the mod's folder (not inside in the "gd_PC" folder.) and then you will need to type these texts:
With that, for the name part change to name of the mod. For the description part, add the description about the mod, i recommend a simple context of the mod. And the finally, for the author, add your normal username or your GameBanana if your uploading this on the website.