Let me say you have a Percentage metrics.
you want to bucket it as A,B,C,D such that
<25 - A
you need to make use of the metrics Percentage column and click Edit formula and use switchcase in expression and write something like
when Percentage <25 then 'A'
when 25<Percentage <50 then 'B'
when 50<Percentage <75 then 'C'
else D end
this will bucket the percentage values in A,B,C & D.
hope this helps. this you can do it in reports and rpd as well.