Benzene -> Nitrobenzene -> Aniline -> N,N-Dimethylaniline
Step 1: Sulfuric and Nitric acid, described at the hive, UTFSE
Step 2: Reduction with iron or tin, described at the hive, UTFSE
Step 3: Eschweiler-Clarke reaction (formaldehyde/formic acid), described at the hive, UTFSE