When & how to tell?

I met my DH when I was two months pregnant with my daughter. He went to all appointments with us and was there at her birth. Everything a daddy does. BF was in jail for assault at the time. My daughter had my maiden name (which made the adoption go really easy for her because she thought when we got married we both got new names) BF visited for first year then lost job, interest and everything else to drugs. We waited as long as we could for the adoption to give him a chance to change his life. We were married in Jan so that we could get the adoption done before she started school the following Aug and she would have her new name. We've used the adoption word around her little by little but I'm not sure how to explain it without going into the birds and the bees. She thinks god put her in my stomach and does not understand the it takes two people idea nor am I ready to explain all that she just turned 6. I worry all the time about how to tell her and when to tell her. I've even tried to find some books for around her age but haven't found ones i felt worded it right. Does anyone have suggestions?