To get started with the multi project custom field, the Jira user has to have admin privileges
Instructions
To create a multi project custom field
Click on the cog that is visible on the top right corner of the nav-bar.
Choose Issues
Then from the side-bar select Customfields
Click Create
Go to All and look for Project Picker (multiple Projects)
Name the newly created customfield
Voila! You have successfully created a Multi Project Custom Field