If Earth's axis were not tilted, there really wouldn't be any seasons. There is one condition that is necessary. There must be substantial land masses in the high latitudes where ice sheets and glaciers can form. This is the case now with North America, Europe, and Northern Asia. If the land masses are available, the easiest way to have an ice age is to decrease the contrast between winter and summer.
Warmer winters means that it snows more, because warm air can hold more moisture.
Cooler summers means less snow melts, and hence the glaciers can grow. Milankovitch Cycle Earth's orbit is not constant. Over a period of 90,000 to 100,000 years, Earth's orbit stretches into a longer ellipse and then returns to a more circular shape. The tilt of Earth's axis is also not constant. It varies between 22.1 and 24.5 during a cycle, which takes 41,000 years.