a small hole in the soil. Use a trowel or spade to dig a hole 2–4 inches (5.1–10.2 cm) deep. Break up the soil within the hole and remove any twigs or foreign debris. Use distilled (not spring) water. You can find this in your local pharmacy. Rainwater is slightly acidic, and bottled or tap water tends to be slightly alkaline. Fill the hole until you have a muddy pool at the bottom. Make sure your tester is clean and calibrated (for a more exact measurement). Wipe the probe with a tissue or clean cloth, and insert it into the mud. pH is usually measured on a scale of 1-14, though the tester may not include this entire range.  A pH of 7 indicates neutral soil. A pH above 7 indicates alkaline soil. A pH below 7 indicates acidic soil. A single reading may be an anomaly, so it's good to get an idea of the average pH in a plot. If they're all around the same, take the average and amend the soil accordingly. If one spot is very different than the rest, however, you may need to "spot treat" it.

TL;DR in Vietnamese:
Đào một cái hố nhỏ trên đất. Đổ đầy nước vào hố. Chọc thiết bị kiểm tra vào chỗ bùn. Giữ nguyên trong 60 giây rồi đọc kết quả. Đo ở nhiều vị trí khác nhau trong vườn.