1. press ctrl- a ( cmd-a for mac) <--- at this step, its the
same as select>all
2. press alt-ctrl-d( apple-cmd-d for mac)<-----at this step, its
the same as select>modify>feather.
3. press ctrl-shift-i ( cmd-shift-i for mac)<--- at this step
is same as select>inverse
4. input the feather radius u want. eg 10.
5. press delete and u are done