UE4 - How to create Nebulas and Clouds using Niagara´s particle system?

I wonder 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