Start with the basic particle system presented in the Creating a particle system using transform feedback recipe:

1. Set the uniform variable Accel to a small upward value like (0.0, 0.1, 0.0).
2. Set the ParticleLifetime uniform variable to about 10 seconds.
3. Create and load a texture for the particles that looks like just a light-grey smudge. Bind it to texture unit zero, and set the uniform ParticleTex to 0.
4. Set the MinParticleSize and MaxParticleSize uniform variables to 0.1 and 2.5 respectively.