1. Open Measurements and pick a popular one (Bodyweight, Waist…) or Something else… with your own name and unit.
2. Open it: your latest reading is at the top, with how much it changed since the one before and over the last 30 days. Type a new one under it and tap Log.
3. Trend draws them all. Tap 1M, 3M, 6M or 1Y to see only the last month to year, or Dates to pick your own from and to. Readings lists the latest five (All readings for the rest).
4. To rename it or change its unit, tap ⋯ → Rename or change the unit; delete it from the same menu.

If you have a coach, switch on Your coaches can see it on a measurement (or pick them in Coaching). They can look, never change.