Genko yoshi, US Letter
Japanese manuscript paper on US Letter, for anyone learning Japanese on a printer that has never seen A4.
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Letter for learners outside Japan
Genko yoshi is traditionally B4 or B5 and almost never Letter, for the straightforward reason that Letter is not used in Japan. But most people printing practice sheets are learners in North America whose printer holds Letter and nothing else. This is the same ruling on a page that will actually feed, which matters more for practice than tradition does.
What the ruling is doing
One square holds one character, including punctuation, and the narrow channel beside each column is for furigana, the small kana that show a kanji reading. That gutter is not margin and leaving it out is the usual error in improvised versions. Standard genko yoshi holds 400 characters as twenty columns of twenty, and Japanese assignment lengths are stated in sheets rather than words.
Vertical first, horizontal if your course says so
Traditional genko yoshi is written top to bottom with columns running right to left, so composition starts at what a left-to-right reader thinks of as the back of the page. Horizontal genko yoshi exists and is used in some schools and for technical writing. Ask which your course expects before you fill thirty sheets. The A4 version is closer to the size used in Japan.
Printing it accurately
This only measures correctly if your printer doesn't rescale the page. In the print dialog set Scale: 100%, not "Fit to page" or "Shrink oversized pages". If you're not sure it worked, print the calibration page, measure the ruler on it with a real ruler, and it will tell you exactly what your printer is doing to the page.