Why not just work inside of Lightroom? If you have Lightroom, simply just import to that App and edit from there. If you want to work on a project inside of iPhoto then export the images out of LR to a folder and then import to Photo.
Here is where it gets fun. You can decide. Everyone here works differently. If you are new to LR, you can check out a few tutorials to get you started.
I like Adam Lerner's quick tips on management inside Lightroom