How do I install applications from a .dmg file?
A .dmg file is kind of like an USB stick in a file and can be handled more or less the same way. To install from a .dmg file you usually do the following:


double click the .dmg to make its content available (name will show up in the Finder sidebar), usually a window opens showing the content as well
drag the application from the .dmg window into /Applications to install (may need an administrator password)
wait for the copy process to finish
eject the .dmg (by clicking the eject button in the Sidebar)
delete the .dmg from Downloads