At an old house I would regularly make a new raised bed when I wanted
an additional garden. Procedure was simple:

(1) go to Home Depot (or similar) and buy 5-10 bags of composted manure
and treated 3x5 boards to make the edge of the bed.

(2) build the frame of the raised bed and then set down newspaper over
the inside bottom of the frame (covering the grass)

(3) cover the newspaper with the composted manure.

the grass will yellow and die within 2-3 weeks and by the following
spring (when you spade things over) the newspaper will have completely
disintegrated. The extra advantage of the paper is that it prevents
the grass from gowing up theough the 6" of manure.