Long vowels are written differently in each script

Katakana is the easy case

In katakana a long vowel is just ー. It means "hold the previous vowel one more beat".

コーヒー is コ + ー + ヒ + ー — kōhī, four beats.

It is a horizontal line in horizontal text and a vertical one in vertical text. It looks almost identical to the kanji 一 and to a hyphen, but inside a katakana word you can safely read it as a long vowel.

Hiragana adds a vowel kana

Hiragana does not use ー. It appends a vowel character instead.

Here is the trap

Look at the e and o rows. You write い and う, but you say a long e and a long o.

せんせい looks like "sensei" but is pronounced "sensē". おとうさん is not "otousan" but "otōsan".

This catches more beginners than anything else in kana. Pronouncing the written i and u distinctly is an instant giveaway.

There are exceptions: ええ (yes), おおきい (big) and とおい (far) double え and お instead. There are not many, so learn them as you meet them.

Writing them in romaji

Romaji has three options: a macron (kōhī), a doubled vowel (koohii), or a hyphen (ko-hi-). Textbooks differ, so knowing one is enough.

Note that plenty of everyday romanization simply drops the length — Tokyo for とうきょう (Tōkyō), Osaka for おおさか (Ōsaka). The names you already know are usually the shortened spellings.

The chōon practice here accepts a macron, a doubled vowel or a hyphen, all three. The point is to make you notice the length at all.