In your document use the Zotero toolbar to set Document Preferences and indicate that you want to use the custom style for this document
In Zotero, go through each item and add a word or a number to the "Extra" field. The custom style will group items with the same word/number together and then display the groups alphabetically/numerically in your bibliography.
Finish your paper and add your basic bibliography.
Save a copy of your paper.
Click "Unlink Citations" in the Zotero toolbar of your word processor
Add section headings to each grouping of citations