A means to hold the document or photo exactly perpendicular to the camera lens for low distortion and perfect focus over the whole document. The document or photo should also have as flat a surface as possible without damaging a curled object by bending it too much. Another person can hold the object in position, if they brace it against something solid to keep it from moving even a little bit. Otherwise you'll need some kind of jig made up ahead of time to keep the object being photographed rock steady. A good light source.
Indirect sunlight is excellent.
An Ott light works well, too. Or, in a pinch, use any bright, diffuse light available. Once all this is amassed, mount or otherwise orient the object being photographed so that it is well lit and stable. Now focus carefully and frame the object dead center to fill the view as much as possible e.g. get as close as possible so you pick up the most detail. Check to make sure no shadows or reflections fall on the object.