I'm a newbie and just did the same thing. My Buff Orphington hen became broody --- only hen, no roo. So, I went to a farm and bought a dozen fertilized eggs. As he said, he didn't know if they were all fertile or even from which of his hens they were.
I came home with the idea of slipping in a few at a time, but wound up slipping all 12 under her. She just kept shifting her feet and "purring." Two broke --- could have been they were from young hens or just my hen's inexperience. So, now she is happily sitting on 10 eggs which should hatch on Fri the 13th.
Since I live in the "city," I have to limit myself to 4 hens ... but I figure that not all will hatch and 1/2 will become cockerels and need to go elsewhere.
Good luck!
Eileen