Open a video window, Tasks / Capture / Capture Video Frame, and when it shows the desired area, save the file. The choice of file name is a question of personal preference. We find it helpful to use a format that indicates where in the mosaic the data comes from -- for example, dat3.5.v for the fifth image in row three.
It is helpful to be able to see the previous image to ensure that there is sufficient overlap. To achieve this, a second video window can be opened and placed alongside and the images grabbed into alternate windows.