What type of bale?
Small squares- First layer straw then stack so the bales interlock and tie stack together. On top lay a row of bales in the center of the stack and put the tarp on and secure it snugly. That allows the top to shed water and snow. If you live in windy areas, invest in a real hay tarp because a blue tarp won't cut it.
Rounds with twine tie - Stack the first one vertically and then the next row horizontally. I've seen people go two vertical and then one horizontal. I don't.
Rounds with net wrap- stack on the ground butted together with each row separated by thee feet. The first year I had net wrap I stacked them like I do twine and the water ran off the net wrap into the lower bale and I had frozen moldy hay in verticle stripes on each side of the bottom bale. Never did that again! My theory is the net wrap allows water running off the top bale to stay attached to the top bale beyond the mid line of the horizontal bale and then drip into the bottom bale. On a twine bale, the rough outside lets very little water beyond the mid line.