In the Southern Hemisphere, the constellation of the Southern Cross (aka Crux) can be used to guide you in a southerly direction. Five stars make this constellation, and its four brightest form an angled cross. Find the two stars that make up the lengthwise portion of the cross and imagine a line that extends out four and a half times greater than the full length of the cross. When you reach the terminus of that imaginary line, draw another imaginary line that extends to the ground. This is, in general, the direction of south.  When the cross is lying horizontal, the "four and a half times" mark will be less than one degree from true south.  When the cross is vertical or upside down, the mark will be about two and a half degrees (about a thumb's width) from true south. Once you’ve determined the general direction in which south lies, it might be helpful to find a landmark at that location on the ground so that you don’t lose where south lies.

How would you rephrase that briefly using Vietnamese?
Nhận biết chòm sao Southern Cross (Nam Thập). Dùng chòm sao Nam Thập để tìm hướng Nam. Chọn một cột mốc.