We need at first an emitter, so let’s for example add good “Suzanne” to the scene:

Add a “Particle Mesh Emitter Node” to your “Simulation Tree”:

Connect the two yellew “Emitter Sockets” of the two nodes in your “Simulation Tree” and select “Suzanne” from the “Object” list in the “Particle Mesh Emitter Node”:

Press SPACE and Suzanne starts emitting particles:

With the “New Particle System”, you can change the “Emission Rate” and set “Keyframes” in order to animate this setting (!):
