Each player loses a third of their life, then discards a third of the cards in their hand, then sacrifices a third of the creatures they control, then sacrifices a third of the lands they control. Round up each time.
The losses, discards, and sacrifices are rounded up. For example, if you have 5 life, you’ll lose 2 life, leaving you at 3 life. (Five divided by three is one and two-thirds. Rounding that up to the nearest whole number gives you two.)