These are the simple prefixes used in conversion.
Like centi is 10^-2
milli is 10^-3
micro is 10^-6
1m = 1000mm
or 1mm = 10^-3m
1g = 1000mg
1mg = 10^-3g
now let us say you want to convert 100 s to ms
then use dimensional analysis
= 100 s X 1000 ms / 1 s = 100,000 ms
or
= 100 s X 1ms/ 10^-3 s = 100,000 ms