UNREAL UE4 - Niagara´s particle system - How to create Clouds and Nebula?

Hi, how to create clouds and Nebulas with Niagara? the main problem i have is that i canot freeze particles, they stay moving, also as the game starts, particles start to be generated… and i Will need them to already be set in position from start, have you found a video tutorial for Clouds or Nebula with Niagara?

Please help