Here's where they appear, depending on where you've progressed in the game:  At the Aircraft Carrier in San Fierro. In the "Restricted Area" in Las Venturas. At the abandoned airstrip north of the Restricted Area after completing the mission "Vertical Bird."  Keep your finger on X:A. Use the left analog stick to control the Hydra. Ascend to a point where there are no obstacles. Press R3 to bring the wheels up and slowly push the R stick forward. This will send the Hydra in a forward direction. To land, you must do everything in reverse. Pull the R stick towards yourself to rotate the engines downward, for "hover mode," use Square to slow the Hydra down, and press R3 to lower the landing gear. Press R1/RT to lock on. Press the alt fire button to shoot.

Summary:
Find a Hydra. Enter the jet by pressing Δ:Y. Get the Hydra into the air. Fly forward. Land. Shoot targets.