The start up screen of S_Merge is the 'Align' window. This is where both images are loaded and aligned together.

Use the left/right (for source 1 & 2) mouse buttons to drag corners, edges and the image itself to scale size and position it. Total flexibility of overlaying the two sources is allowed insofar as they can be completely overlaid, partially overlaid or totally separated.

In the Align menu you also have the options to rotate either of the sources and mirror them in the vertical plane.

This window is autoscaling is so far as the blended image size is roughly a fixed proportion of the Align window size. This feature makes it easy to align and size images without them going offside or shrinking out of site. To get an idea of the final real size, look at the predicted target size display at the bottom of the main window.

When finished with alignment, switch to the target window. A quick way of doing this is pressing the green button on the bottom right.