There's a detailed instruction in the Ultra Microscope software manual:

in short:

- set up grid size
- set up same overlap that is used in Imspector
- select input directory
- set 'First File index' for x and y to 0

Copy first file name of the series and change the position markers from:

     [00] x [00]
     {yy} x {xx}

When copying the file name make sure to copy the complete name incl. the '.tif' file extension!