To unlock the Professional Dumper trophy in Cities: Skylines, you will need to completely fill five landfill sites.
You will unlock landfills sites very early in the game, once you reach the Little Hamlet milestone for having a population of 120 - 480 depending on which map you start on.
The trophy will unlock once you fill five of them (simply building five is not enough). The higher your population, the quicker this will happen.
To speed up the process:
- Don't buy a garbage incinerator
- Don't use the recycling policy