Leviathan was one of the first raids that I did, it is complicated for a newbie and understanding it can be difficult. The underbelly of Calus is something so big, that is requires a map, so the community artists have created an amazing graphic design to make it easy for everyone.
Leviathan Underbelly Map
Right click > Open image in new tab. This will open the image in full resolution.
Do let us know in the comment section below if this helped you out.