Hiragana and katakana practice paper
Practice squares with a faint cross through the centre, 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 centre
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 centre and the quarter points, which is all the reference the eye actually needs to place them.
Fifteen millimetres 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.
Practise 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 practise 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.