From The Document Foundation Wiki
- place the cursor at the end of a part, on the bookmark:
- open the Navigator by double-clicking on its name;
- right-click in the status bar on the area where page numbering is displayed, a dropdown list will display the bookmark names and you can reach one by clicking on its name in that list;
- select ▸ ▸ ;
- name it if you wish (useful for the Navigator for example);
- check ☑ Protect under Write Protection (you can even add a password);
- check ☑ Hide under Hide;
- click Insert.