How to catch Nincada
Nincada is perhaps the hardest Pokémon to catch in the Hoenn Collection Challenge simply because it's only available in one way. Nincada will have increased spawn rates in the wild, but won't be available in raids or even eggs, which wouldn't count towards the collection challenge anyway. Still, rarity will likely be frustrating for many players, so try to keep an eye on your nearby roster to see if one spawns somewhere around you.
To increase the chances of catching a Nincada during the Hoenn Collection Challenge, you'll probably want to use both methods of increasing Pokémon spawns. Have incense active whenever you play the game for long periods of time, and activate a decoy on any nearby Poké Stop if you can. Many people have already proven that these methods cause more Nincada to appear, and besides, they are also a good way to get Nosepass.
How to catch the Nosepass
Nosepass is almost exactly in the same situation as Nincada, as it is only available in the wild during the Hoenn Collection Challenge event. However, it's much more common, and you'll likely be able to spot one on Nearby if you're vigilant. However, if you don't see one in the first day or two, you might want to use the same methods listed above to get one. It's about using incense when possible and activating a decoy on any Poké Stops you have access to for a long time. This will increase the chances of seeing Nincada, Nosepass, and all other Hoenn Collection Challenge Pokémon whose wild spawns have been boosted.
And that's how to catch Nincada and Nosepass for the Hoenn Collection Challenge in Pokémon GO. For any other help you need, see our general guide to Hoenn events.