If your fair is integrated with 4HOnline, you can require that members be enrolled in projects (on their 4HOnline record) before they can enter divisions in your fair. This process is not applicable to fairs that are not integrated with 4HOnline, and it does not constrain entries by non-4HOnline exhibitors (FFA, open).
The first step to requiring project enrollment is to enable that entry setting. The most common place to select that option would be at the Fair level of the hierarchy. The entry setting "Require 4-H Project Affiliation for 4-H Exhibitor" is in the group "4HOnline Settings" near the bottom of the Entry Settings list. The default is "No" and you will need to change it to "Yes" and save.
You must import the projects from 4HOnline via the Setup>>4-H Integration tab before you can go on.
You will then need to map each division to its corresponding project(s). Select a division from the Hierarchy screen, then click on the Hierarchy Editor tab to attach a project from the drop-down list. Click Save when all projects for that division are selected.
Continue to the next division, mapping each division to the correct project(s).