The basic process is:
> Do a clean install of D2+LoD (version doesn't matter).
> Download and install the MXL Sigma Launcher (click the "Download" button in the upper-right corner of the forum page). It will take care of the rest.
> You'll also need Windows 7 or higher. Neither Sigma or the launcher support WinXP or Vista.
I'm not a Windows user, but I think you'll also need to add exceptions to whatever Windows' native security feature is for "Diablo II.exe" and "Game.exe". You may also need to add exceptions to your antivirus program.
If there's more to it, I'm sure someone else will chime in.
Edit:
I just noticed this topic exists:
viewtopic.php?p=206270#p206270This is probably what you're looking for (scroll up to the top).