Walk the boundary and watch the area build. Your device's live GPS accuracy is shown throughout, because it is the number that decides whether the result means anything.
Press Start, then walk the boundary. A point is recorded as you move.
A phone's GPS is typically accurate to a few metres in the open, and considerably worse under trees, beside a wall or between buildings. That error applies at every point you record, so on a small plot it is a real share of the total area.
The accuracy figure above is what your device reports, live. If it is worse than about ten metres, the area is not worth much — move into the open and wait for it to settle.
GPS accuracy depends on your device, environment and satellite conditions. This measurement is approximate and should not be treated as a legal cadastral survey.