One quick note: Before you migrate all of your existing photos into Photos for OS X, make sure you have enough i Cloud storage space if you want to take advantage of i Cloud Photo Library, which syncs your images across al of your devices.
With i Cloud Photo Library turned on in the Photos app, all of your images will be uploaded to the cloud, which does eat up your i Cloud storage space.
With the launch of OS X 10.10.3 Yosemite, Apple released its much-anticipated Photos app, which was first announced during its Worldwide Developers Conference in 2014.
The Photos for OS X app took several months to complete because it was designed from the ground up to work with OS X Yosemite and integrate with both the Photos for i OS app and i Cloud Photo Library.
First select the largest library and wait for it to completely migrate from i Photo or Aperture (this could take a very long time, depending on how many pictures you have in your largest library).
Then, you can begin the process of migrating other photo libraries.
There really is no easy way to tackle a gazillion photo prints, negatives and digital photos.