Please explain me how many minutes before the exact begin of zuhr or any obligatory prayers are nafl to be offered at? As these days we do not use the shadow indicator and also it’s unpredictable for me to offer nafl prayers.

Time for Nafela e is from awwal e waqt of dhuhr therefore first pray nafela e dhuhr then offer your dhuhr prayer
You can not say nafela e before dhuhr time