You need a weidu.exe in you bg2 folder. If you don't have one, copy one of those Setup files coming from a mod (for example Setup-BGT-Main.exe) and paste it. Name that copy "weidu.exe".
Now, once you start the command prompt, you need to type all the cd command between double quotes because there are spaces in the path.
cd "c:\program files\black isle\bgii - soa" (and then Enter)
(if you haven't used the default folder for installation, please modify the command above accordingly).
Your prompt will now be C:\Program Files\Black Isle\BGII - SOA> _ or something like that.
Now you can type the weidu thing like above.
Don't forget to hit Enter after each command.
Good luck.
Wouldnt it be just as easy to recommend people not familar with good old dos to make a batch file
and save that in the bg ii folder as a txt file