Easiest way I can think to do this would be to use the PowerPoint equivalent for Open Office. Use a table with no color and all borders over the top of the image.
- Insert the picture of the map
- Then insert a table, setting the number of rows to the number of squares tall and the columns to the number of squares wide
- Turn the table color off and turn on all borders (2 point width or so should be easily seen)
- Change the table size to 1 inch wide per column and 1 inch tall per row.
- Move the image to fit under the table
- Resize the image to fit the table by dragging the corner control points while holding Ctrl (I think - PowerPoint uses shift to constrain the proportions, Libre Office seems to use Ctrl).
Eg. Creating a map for a room 40' wide by 30' tall room would need a table 4 columns wide and 3 rows tall. The table size would be 4" wide by 3" tall.