How do you locate a spawned Prismana?

The practical upshot is that you search the sky, not the ground. A spawned Prismana announces itself with a column of light coming down onto its position, so the moment one appears you stop scanning terrain and start looking for the beam. Then you run to its base.

There is a second marker if you want a direction rather than a landmark. Activating the weather change crystal creates a purple beacon that points straight at the spawned Prismana. That is useful when terrain, tree cover or distance is hiding the descending beam from where you are standing.

What puts one on the map in the first place

Two things are worth knowing before you go looking. Landing a Lucky Strike while catching an Aniimo has a chance to immediately trigger Prismana flow, so good catching is also how you generate the thing you are hunting for.

Nurturing is the other route. The base probability of triggering a Prismana appearance during nurturing is a flat rate that does not move across attempts. No pity counter, no ramp. Your tenth attempt has the same odds as your first, which means there is no point grinding a streak in the hope it is building toward something.

You also get a free rare Prismana as a launch reward, so you will have one in hand before you ever chase a beam.

Prismana are not shinies

Prismana variants are distinct regional or alternate forms, and their markings differ from those on shiny variants. So when you reach the bottom of the beam, you are looking at a different form of the creature rather than a recolour of the standard one. Worth knowing if you are trying to identify what you caught after the fact.