It works like this, Autoflowering is either codominant or recessive or somewhere in between. I have no idea if there are variations in this. But when you cross an auto to a photo you will get this:
A = Autoflower
a = Photo
AA x aa =
a a
A Aa Aa
A Aa Aa
Results of that line = Aa exclusively.
From there you can F2 them and it will create a 25% photo, 25% auto and 50% both ratio. The autos will obviously show themselves under veg light and from there you can work with what you've got. The plants that autoflower will all remain autoflowering until they meet another plant that's got photo genes. You can work the line as you see fit. Or you can take one of the better plants that doesn't auto and self it to see if it produces any auto offspring... or you can... you get the point.
Just making something autoflower is easy. Making it good is hard.
By the 3rd generation you will have completely autoflowering plants.
1 = Aa
2 = AA, aa, Aa
3 = AA or aa or Aa or a mix depending on what plants you choose but the ones that are autoflowering will stand out and be easy to select (duh).