Wrong forum.
It has nothing to do with Battle.Net Hacking.

Most Mods come with a readme.txt, or some sort of text file which would go through the steps to install.

Basically there will be a .MPQ file, or something similar. Most Mods will tell you to create a Backup for the original Patch_D2.MPQ and then rename the Mod's .MPQ to Patch_D2.MPQ.

However, you can do it differently by...
Creating a new folder in your Diablo II folder.
Place the Mod's .MPQ into the newly created folder, and rename it to Patch_D2.MPQ
Locate one of the shortcuts of Diablo II.exe, usually on your desktop. Create a shortcut if you don't have one currently.

Copy the shortcut. Right-Click, go to Properties.
The target by default should be "C:\Program Files\Diablo II\Diablo II.exe"
The starting location by default is "C:\Program Files\Diablo II\"
Change it to "C:\Program Files\Diablo II\NAMEOFNEWFOLDER", where NAMEOFNEWFOLDER would be replaced by that folder name which contains the mod's patch_d2.mpq

Run the new shortcut for Mod, run the old one for original.