How should I vibrate the sound of my soprano recorder?
Note: I'm not sure to use the correct term. By vibration I mean producing the sound similar to vibrating your hand on a Violin during playing a long note (a whole note for example).
Since the recorder is a type of flute that doesn't have an embouchure which affects pitch, all you have control over is the speed of the airstream. As a result, recorder vibrato is executed by adding pulses to the airstream at a regular interval. Since the airstream should be driven by the diaphragm, we call this diaphragmatic vibrato.
Locate your diaphragmatic support muscle by laughing, coughing, or by saying the word "Ha!" in rapid repetition. Then, try adding pulses of this muscle to a steady and smooth airstream. Start with a very slow interval between pulses, and gradually increase the speed of the pulses at regular rhythmic intervals, with the aid of a metronome for control.