Install a flexible air duct involves the following steps:

1. Preparation: Ensure that you have performed all necessary calculations, including Manual J load calculation, Manual D for duct design, and Manual T for determining register and grille sizes, before deciding to use a flexible duct.

2. Connection to Main Ductwork: Connect the flexible duct to the main ductwork or equipment using clamps, tape, or other fasteners provided by the manufacturer. Ensure an airtight connection to prevent leaks.

3. Proper Support: When installing flex ducts for straight runs, ensure they are pulled tight and properly supported to maintain their shape and prevent sagging over time.

4. Testing and Balancing: After installation, test the ductwork to check static pressure and airflow through the system. Make adjustments as necessary to ensure the system is moving the right amount of air.

5. Quality Control: Due to the ease of bending or crushing flex ducts, rigorous focus and precision during installation are crucial. A post-installation audit by a senior team member is also recommended.