what's the best temp/time for roasting soybeans to make coffee substitute?

Years back, I roasted soybeans and ground them into coffee. My class at the time loved the coffee this made, and the entire pot was empty at the end of the day.

of course the beans i roasted in the Broiler in the oven, the color was off, and it didn’t taste right.

What did I do wrong, and how can I make it right?