Minecraft wiki entry for Saved_data_Dropbox_guide, associated text: 1 Download and Install Dropbox
2 Move your Minecraft Saved Data to your Mega Folder
2.1 Windows
2.1.1 Alternate (command line) method
2.2 MacOS
2.2.1 Alternate (command line) method
2.3 Linux
2.3.1 Alternate (command line) method
3 Create the Symbolic Links
3.1 Windows
3.1.1 Windows Vista, 7, 8, and 10
3.1.2 Windows XP (after installing junction.exe)
3.2 MacOS
3.2.1 Alternate (command line) method
3.3 Linux
4 Note on Multiple Operating Systems
5 Alternate Method