NavList:

There are five parts to it, a base disk, white, printed on both sides, plus transparent disks on both sides, that rotate on a central pivot, plus index arms on each side that also pivot  to align with the scales on the disks and the base.  

 . You start on the distance side. Look at my prior post. The first photo shows the index on the transparent disk aligned with the 30 degree mark of difference in latitude on the base and the mid-latitude of 25 degrees on the index arm aligned with the 20 degree of differnce in longitude curve on the transparent disk which causes the index arm to align with the distance of 2105 nm on the white base. 

You then flip it over and align the the cursor on the index arm with the 30 degrees of latitude difference, rotate the disk until the distance of 2105 is also under the cursor and then read out the course from the "INDEX" on the disk. In this example (the second that I posted) the "INDEX" points to "WARNING" on the inner course scale. If it had pointed to a number then we would have been done with the computation but now we have to flip  it over again and use the other side to determine "equivalent latitude difference." This time we place the index on the disk to "zero" on the base and the 25 degree arrow on the index arm to the 20 degree curve of difference in longitude and we read out 18.5 degrees as the "equivalent latitude difference." We then flip it over again and align the 18.5 degrees on the base with the 2105 distance on the disk and read out the course of 31 degrees on the inner,  course scale on the base.

https://navlist.net/m2.aspx/A5-Course-Distance-Computer-LaPook-jun-2021-g50817