Want to make it memorable?
Tell her to come home at 6pm sharp. Have candles lit in the house with a note saying to tie this silk blindfold on just before entering the bedroom. Get some Barry White going also in the backround. Then, when she enters, slowly undress her, take her hand and lie down in bed with her on some silk sheets. Pour a glass of red wine and have her drink from it. Then, when she's absolutely relaxed, tell her to pull back the blindfold so she can see her girlfriend going down on her.
It'll be memorable one way or another.