What you want is for your child to know that you are comfortable with adoption, with adoption terminology, with who she is and where she is from, you want her to know she is loved and was loved before.
Here are some ideas from another amom
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1) buy some books that talk about adoption. Katie's Koala is a good one for her age level. It talks about Katie "adopting" a teddy Koala from the zoo and relates it back to how Katie was adopted as a baby.
2) Make her a life book. Use simple pictures and a story that she can follow. If there are no infant pictures of her, draw some together or use magazine cut outs (dont pretend they are her ... explain that this is what you think she looked like etc)
Use the word "birth mother" or "first mother". The reality is that there is not some baby store out there with "tummy ladies" pumping out kids. She has another mother who, for whatever reason, loved her but knew that your daughter needed another family to take care of her. Offer information in an age appropriate way. Using the term mother wont confuse her and it wont take anything away from your motherhood either. Its honest and she will understand.
3) Sounds like you are already doing it - but expose to frequently to other children, adults etc who are from families like hers. POINT OUT when you see other adopted children or kids from Russia etc. She needs to internalize that its ok and "normal".
Some kids dont process adoption until much older, other are far more keen to know and understant. Sounds like you have one of the later. I know how it is ... I have one of each. But I promise you this - you will never regret being honest and open!