How Do You Change 1,500g Into Kilograms?

You did the maths incorrectly..
1500/1000 = 1.5kg, not 15.
But the principle was correct.
1500g = 1.5 kilograms

To be technical, one does not divide by 1000, or 1000 g, but divides by (1000 g / kg), which is the same as multiplying as below:

1500 g * (1 kg per 1000 g) =
1500 g * (1 kg / 1000 g) = (1500 * 1 / 1000) * (g * kg / g) = 1.5 kg