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; 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.