In wedge dash, formula molecular structure i.e arrangement of atoms in organic molecules is depicted by Wedge for front atoms and dash for atoms at the back and rest are in the plane of the diagram. While in Newmann Projection are drawn by looking directly along a particular bond in the system (here a C-C bond) and arranging the substituents symmetrically around the atoms at each end of that bond.

It is not an easy task to explain thumb rule for conversion in detail, But if you look at the NPTEL lectures on conversion You will be able to easily understand this the link for the following is given Below:

https://nptel.ac.in/courses/104103071/module8/lec21/1.html