Which part isn't clear?
Create an empty .xml file, copy the contents of the code-box to it.
Place the .xml anywhere on your SD, navigate to and launch it with the HomebrewLauncher.
or..
I've included a .xml and .smdh with blue xash3d logo, made for blue shift, in the attached archive.
Copy the contents of the archive to: 'sdmc:/Xash3D/', and launch it with the HomebrewLauncher.
(.xml can be placed anywhere, for example: 'sdmc:/3DS/')
Most other mods probably don't work without porting/recompiling, like MasterFeizz already mentioned.