If you don't have a flat table to build on, it is very hard to build the part you want without warping it. If you don't have one, build one.. It saves loads of time.
To make your table flat, build it with many legs. Then use a tight string to define 'flat', and shim the legs until the tabletop matches the string. I neglected to flatten my table, and now my boat has a downward kink in the rear.