To get started and make your own particle type, create a json file at the following location of your pack: addon/<namespace>/particle_types/<particle ID>.json.

The json file itself can look like this:

{
  "lifetime": 60,
  "has_physics": false,
  "gravity": 0,
  "quad_size": 0.5,
  "render_type": "lit",
  "texture_type": "aging",
  "brightness": 240
}

lifetime is the amount of ticks the particle will live on for. 60 means that the particle will exist for 3 seconds. (optional, fallback: 100)

has_physics determines if the particle can collide with blocks in your world. (optional, fallback: true)

gravity determines the gravity factor. 0 means no gravity at all. Positive values will make the particle go down, zero ones will make it go up. (optional, fallback: 0.02)

quad_size determines the size of the particle texture. (optional, fallback: 1)

brightness determines the glow intensity of the particle. Needs the render type set to "lit"! Max is 240. (optional, fallback: -1)

render_type determines how the particle is rendered. Possible values: "opaque", "translucant", "lit", "no_render". (optional, fallback: opaque)

texture_type determines how the textures are handled. More info in the bottom section. Possible values: "random", "aging". (optional, fallback: random)

To determine which textures can be used for your particle, you need to create an additional json file in the assets directory: assets/<namespace>/particles/<particle ID>.json.
In there you specify the particles like this:

{
  "textures": [
    "test:test_particle_0",
    "test:test_particle_1",
    "test:test_particle_2"
  ]
}

Depending on the texture_type you specified above, the textures will be chosen differently. random means that the particle will have one static texture during its lifetime, but it will be one random one from that list. If you choose aging, the particle will go from the top to the bottom texture through its lifetime.