The scanning plane (imaging plane) should be parallel with the plane of motion of the X and Y stages. You will need to mount the stages on three points of contact with at least two being height adjustable. Image a thin fluorescein layer under a coverslip. Translate the stages two or three mm. The image should not change. If it does, the slide is tilted and you need to re-mount it. Assuming the image plane is not curved, the whole FOV should snap into view at once as you focus through it. If there is field curvature you will likely see the centre appear first and as you move the objective further down you will see a donut. If the stages are aligned with the image plane, the donut should be symmetric.