Very simply you open a command prompt and change to the folder where adb is located (tools or platform-tools, depending when you installed the SDK).
With USB Debugging enable on the phone and the phone plugged into the PC, type:
Just to ensure the phone is recognized. It will be listed as a serial number
Then to install your .apk file type:
adb install <full path to .apk file>
It will push the app to your phone. When done, unmount the phone and it will be installed.