The easiest way to get an image that looks straight on is to photograph it from as high as possible using a longer focal length, this greatly reduces the effect of not being quite parallel, and prevents objects in the corners from appearing to bulge outwards.
I usually use a tripod set to maximum height and a 100mm lens and shoot through the legs. If you have an indoor balcony from a floor above that would be even better.
Flat lighting is a matter of having a very soft lightsource. Window light from patio doors works well, or a flash bounced off a plain white ceiling.