You take first 30 mins candle and use the high and low of that candle to form a box (as shown in 2 blue lines in the chart).
Most of the time, this box is the trading range of your stock for the day. But if you see stock move aggresively towards and beyond the high bar, you will likely get a uptrend for that day. If you see stock move aggressively towards and below the low bar, you are likely getting a downtrend for the day. Otherwsie, stock will tend to bounce up and down around this box