Mounting and aiming weapons
Weapons like the 
Rotary Cannon, 
Heavy Crossbow, and 
Bomb Bay are separate blocks you attach to your aircraft's 
hull. Place them on a hardpoint, then board the aircraft. While flying, aim with your view — the weapon fires in the direction you are looking, so you steer and aim at the same time. The Rotary Cannon needs ammunition loaded into it before it will fire; right-click the cannon block with your ammo item while dismounted to load it. The Heavy Crossbow works similarly but fires bolts.
Dropping bombs
A Bomb Bay is mounted underneath the hull and drops its payload straight down, so you need to be above your target. Load the bay with bombs while on the ground, take off, fly over the target, and trigger the release control. Because bombs fall with gravity, release slightly before you are directly overhead. Practice on empty terrain first — a mis-timed drop near your own base will hurt. Keep some altitude so you are out of range of your own explosions, and remember that a heavily loaded bomber flies slower and climbs worse than an empty one.