Bottom contours have various purposes, Greenlight Surf Supply has an excellent guide along with guides to all aspects of surfboard design. My experience riding a strapless kiteboard is that having a little concave in the bottom helps with lift while planing, adds a little grip when edging and reduces slapping when going fast.
For the X6, I’m going flat in the nose, concave through the body and flat in the tail. I’m still trying to talk myself into trying out extending the concave into a double concave in the tail but I’m leaning to leaving it as is.
I find it helps to mark out the boundaries. Then I’ll start at the center of the concave and take the stringer down a little with a trim plane.
From here, I sand with 100 grit focusing at the center and working out. When the foam and string are flush, I’ll lay a level across the board, measure with depth calipers and repeat until I’ve hit my desired depth at the center. I’ll also spot check up and down the stringer to make sure everything looks even.
Before 
After-ish
Repeat the process until you’re happy with the contours.