Sure Carolyne . . .
I will post several ideas - up to you to choose which works best for you in your knowledge and budget . . .
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Here is one example short video:
https://www.youtube.com/watch?v=phrSy-6Y2eQthe following tutorial is a WAYYYY over the top over the top example of how to make a Darth Vader voice which you would NOT want to do but will give you an idea of how different filters can be used:
https://www.youtube.com/watch?v=-dehk1ALxFYHere is an example of an all in one plugin that does it
http://www.waves.com/plugins/ultrapitch?gclid=CIip6qCq9ssCFRCOaQodj7gA3QHere is another:
http://www.waves.com/plugins/doubler- - - - - - - - - - - - - - - - - - - - - - - -
Here is a decent one:
http://www.antarestech.com/products/detail.php?product=Harmony_Engine_Evo_4*** Of course if you use the Cadillac Melodyne, you can make harmonies this way:
https://www.youtube.com/watch?v=Noa7efPJVK0Melodyne can be used as stand alone or as plugin in your DAW
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If you want to work out the harmonies individually for cheap or free, I consider the following tool indispensable:
http://www.hooktheory.com/ - I subscribe to this service - but you can use it for free as well . . .
My singer is real good at figuring out his own harmonies, but when we have problems, this tool really helps . . . .
Hope this information doesn't bog you down . . . . sometimes when I try to help, I am TOO helpful . . .
I just tried to give you lots of options . . . harmonies are a vocal effect just like filters are a vocal effect . . . you will have to decide which is for you
I prefer well done harmonies over a vocal effect and a vocal filter over pitch shift harmonies (although that being said, melodyne is remarkable in what it can do)
at the end of the day, a good real harmony almost always trumps anything else . . .
Blessings!