Hello and welcome!
Before I go through all of the 12 borders and what words you need to master for them, I'll first explain how to actually master words.

To master a word, you will need to find it 4 times in a row in "Find Mode". When guessing a correct word, it flashes green. After 4 times in a row however, it will flash golden and you will unlock a border.

If you struggle with mastering a word, mark the word you want the border of as the ONLY favourite, then play Find Mode with the Word Set "Favourites" 4 times.
Since it's the only word you're being given to, you can't make any mistakes.

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Now, to make things more accessible, each border will get its own section in this guide.
Also, borders are listed in alphabetical order.

I hope this guide helps you unlocking all of the fantastic borders!

Unlocked by mastering:

"Cat" / "猫"

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Where to find:

There are a total of 5 Cats in the game. The easiest to find however is the one in the park.

Unlocked by mastering:

"Ghost" / "おばけ"

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Where to find:

Since this is a secret, all you get is the following screenshot.

Go find Timothy for yourself.

Unlocked by mastering:

"Hotel" / "ホテル"

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Where to find:

Go into the park, from there you should be able to see the Bakery. Right next to the Bakery you can find a small passage. Follow it to the end and you'll find the Hotel on the left.

Unlocked by mastering:

"Movie Poster" / "映画ポスター"

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Where to find:

The fastest way: Enter the pause menu by pressing ESC, click "Return To Station", go out of the Station and immediately turn right.

Unlocked by mastering:

"Balloon" / "風船"

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Where to find:

Near the "クリア" Arcade, there's the Hobby Shop building, including the Maid Cafe. The Balloons can be found right at the entrance.

Alternatively, you can also use the "Return To Station" option in the pause menu, so you're already facing towards this building.
You can't miss it.

Unlocked by mastering:

"Truck" / "トラック"

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Where to find:

Near the "クリア" Arcade, there's a small passage. follow it and you'll see right away there's a Truck currently unloading and blocking the passage.

Unlocked by mastering:

"Ramen Restaurant" / "ラーメン屋"

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Where to find:

From the main Station's entrance, turn around and walk towards the Bus Station. Right across the street you can find the "Raianken" Ramen Restaurant.

Unlocked by mastering:

"Manga Comic" / "漫画"

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Where to find:

From the main Station's entrance, turn around and walk towards the Bus Station. To the left, you will find the Book Shop - including two bookshelves full of Manga Comics.

Unlocked by mastering:

"Traffic Cone" / "信号機"

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Where to find:

Traffic Cones can be found basically everywhere, but mostly where map borders are. I'll include a screenshot of the Traffic Cones blocking the Construction, right across the Hotel.

Unlocked by mastering:

"Car" / "車"

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Where to find:

I don't think you won't be able to find a Car, right?

However, if you need a non-moving Car, check the Parking Lot next to the Hotel.

Unlocked by mastering:

"Sticker" / "ステッカー"

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Where to find:

Stickers were placed all around the city. Look for them on Vending Machines, fuse boxes, or just visit the Parking Lot again and look for the Fare Adjustment Machine.

Unlocked by mastering:

"Camera" / "カメラ"

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Where to find:

There are several Cameras placed around the map. However, the easiest to find is the one right next to the "クリア" Arcade. The same one can be found in a box near the Electronics Shop, a different one can be found in the park, right next to some Sweets.