You can add bullet generators to your levels from the control panel by clicking Tools, and then Bullet Generator.
When a bullet generator is selected (shown above right), its properties can be edited.
Bullets per minute: The number of bullets per minute to be generated for each color. Setting this to 60 will generate (on average) one bullet per second.
Speed: Slider to the left = mostly slow bullets. Slider to the right = mostly fast bullets.
Limit to top of screen: When checked, the bullet generator only allows objects above the middle of the screen to generate bullets. Squadron objects can always generate bullets.
Insert generator at: Insert a new bullet generator at the current script time.
Bullet generator script: This allows you to see all bullet generators in the level. Click one to select it.