Sloping ditch for corrugated pipe
For an even slope.
set-up profiles (frames, or stakes) at each end and run a sloping string line between them. Measure down to the floor of the ditch from the string line with a stick of appropriate length.  You could possibly tie one and of the string to the downpipe. and the other to a stake
If you don't need an even slope just use a spirit level (repeatedly) to confirm that the floor of the trench is still sloping as you are digging it.