If your fair is integrated with 4-H Online, you can require that members be enrolled in projects (on their 4-H Online record) before they can enter divisions in your fair. This process is not applicable to fairs that are not integrated with 4-H Online, and it does not constrain/restrict entries by non-4-H Online exhibitors into a fair that has both 4-H and FFA (or open) exhibitors.
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 "4-H Online Settings" near the bottom of the Entry Settings list. The default is "No" and you will need to change it to "Yes" and save.
The project list must be imported from 4-H Online via the Setup>>4-H Integration tab before you can go on.
Each division will need to be mapped 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).
The project mapping (to divisions) will need to be re-established each year, as 4-H Online integration items are not able to be copied over from year to year in the fair.