The setup
Hard floor against a bare wall, child barefoot in light clothing, heels and shoulders touching the wall, eyes level (Frankfort plane), a hard book or square lowered flat onto the head, mark the wall, measure with a steel tape. Morning measurement preferred — spines compress ~0.5 inch through the day, which is a third of a year's growth.
The cadence
Birthdays and half-birthdays — no more. Between-measurement error runs ±0.25–0.5 inch; measuring monthly mostly tracks your technique, not your child. Over 6 months, real growth (1+ inch) outruns the noise, which is the whole design goal.
What to do with the series
Run each pair through the velocity calculator, keep the series in one place (photo of the wall marks works), and bring it to well-child visits. Home series make parents better observers and pediatric conversations more specific — a legitimate, evidence-friendly upgrade to "he seems taller, right?"