Open the Measure app on your iPhone
Move the iPhone slowly to let it detect the area
Point the camera at the object or space you want to measure
Follow the on-screen instructions to place the starting point
Tap the plus button to mark the first point
Move the iPhone to the end point
Tap the plus button again to mark the second point
Read the displayed measurement
Tap the shutter button to take a photo of the measurement if needed
Use the level feature in the Measure app if you want to check alignment
Make sure the object is well lit and clearly visible
Keep the iPhone steady while measuring
Recalibrate by restarting the app if measurements seem inaccurate