1. Install Java and add it to your PATH (e.g. C:\Program Files (x86)\Java\jre7\bin)
2. Download http://rtyley.github.io/bfg-repo-cleaner/ into your Downloads folder (for this example).
3. Follow the Usage instructions at the above link. E.g:

cd "C:\Users\BigFont\Downloads"
git clone --mirror https://github.com/bigfont/orchard-cms-modules-and-themes.git
java -jar .\bfg-1.11.7.jar --delete-files Settings.txt orchard-cms-modules-and-themes.git
cd .\orchard-cms-modules-and-themes.git
git reflog expire --expire=now --all
git gc --prune=now --aggressive
git push

Gotchas

• BFG is case sensitive (I think). I.e. if you put settings.txt it will not delete Settings.txt
• If you want to delete something, it must NOT be in your current commit – i.e. your current commit MUST BE CLEAN.

Todo

• Settings.txt
• App_Data
• Media

See also

• https://help.github.com/articles/remove-sensitive-data
• http://rtyley.github.io/bfg-repo-cleaner/
• http://stackoverflow.com/questions/15796855/java-is-not-recognized-as-an-internal-or-external-command (as usual, Jon Skeet’s answer is very thorough).