You can do this most easily using OS X's built-in accents. For example, pressing Option-e and then "e" again will insert an accent on an e.
What I'd suggest is to go to System Preferences (from the Apple Menu) and choose "Keyboard".
Enable "Show Keyboard & Character Viewers in menu bar"
Click on "Input Sources..." and choose the source keyboard layout to use.
From the menu bar open the Character Viewer or Keyboard Viewer to help you find the shortcuts to insert accents on your letters.
Best thing of all this works across all text input fields in Mac OS X.