I want to offer discount for subtotal from ...

> I want to offer 3% discount for subtotal from $300~$999 and
> 5% discount for subtotal from $1000 or more. How do I set
> this up?

You can do this in "Discounts" menu. For the first discount, set "300" in the "Order subtotal" field and "3" in "Discount" field. For the second discount, set "1000" in the "Order subtotal" field and "5" in Discount field.