Come armed with your Yukata and some spare gil for scoops, and get ready to scoop, scoop, scoop! Head to the area of your choice and talk to the NPC. There, you will receive a free Goldfish Bowl (last option when you talk to them), and have the opportunity to purchase super scoops at 100 gil apiece (they stack to 12). Equip the Goldfish Bowl (range) and the Super Scoop (ammo), then go wading into the water.

NPCS:
East Sarutabaruta, Kesha Shopehllok, (H-8) (sitting on the bridge)
Rabao, Mei, (F-7) (sitting against water pump)
South Gustaberg, Fish Eyes, (E-8) (by Fumaroles, sitting against a rock)
West Ronfaure, Saradorial, (G-10) (at Knightwell, by fence)

This pretty much works just like fishing. You even type /fish (or fish from your menu) to "scoop". However, the wait time in between scooping is much shorter (and the animation more amusing).
When you attempt to scoop a fish, you will get one of these four messages:
A tiny goldfish has approached!
A plump, black goldfish has approached!
A fat, juicy goldfish has approached!
There are no goldfish to be found...

If a fish approaching, you must press enter, just like in fishing, in order to catch the goldfish. After you've pressed enter, you'll have one of these four messages:
Obtained: (fish)
You lost your catch.
The paper on your super scoop ripped.
The goldfish slipped off your scoop...

After you've acquired some goldfish, bring them back to the NPC and trade them. They will count up your fishies, and award you the allotted points.
Fishies:
Tiny Goldfish: 1 point
Black Bubble-Eye: 2 points
Lionhead: 10 points

Prizes:
5 points: a Super Scoop
15 points: a bag of fireworks
25 points: large fans
30 points: a colored drop
50 points: a mystery box (12 Spirit Masque)

Note: you can scoop whether or not you have the Lord's/Lady's Yukata. In fact, whether you have a Yukata or not. But so far, in my experience, I have a much better success rate with my Lady's Yukata. I also think the best place to scoop is East Sarutabaruta, because a) you can get fishing enhancement (in case it helps) and b) it's MUCH easier to spread out there instead of everyone clustered at water's edge.