Hiragana and katakana practice paper
Practice squares with a faint cross through the center, sized for kana. The same grid Chinese practice uses, because kana are written to fill a square for the same reason characters are.
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Kana want a square with a center
Japanese kana are written to fill a notional square, and the hardest part for a beginner is proportion rather than stroke shape, な and た go wrong because the parts are misplaced relative to each other, not because the individual strokes are bad. A square divided by a faint cross gives you the center and the quarter points, which is all the reference the eye actually needs to place them.
Fifteen millimeters is a working size
Japanese practice books for children use squares of around 20 to 25 mm; adult learners generally work smaller. Fifteen is large enough that a pen can articulate the difference between ぬ and め, and small enough to fit a useful amount of practice on one sheet. Drop to 10 mm once the forms are secure, or go to 40 mm and beyond if you are working with a brush pen.
Practice words, not columns of one character
Filling a row with twenty copies of し drills the stroke and nothing else. Kana only become useful in combination, and the spacing and balance of a character shift subtly depending on what sits beside it. Write real words across the squares, one kana to a box, and you practice the letterform and the reading at the same time, which is also considerably less dull.
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.