Performing cunnilingus upon your lover is one of the most intimate acts between two people. The giving and receiving of this pleasure draws both closer. The assumption of nakedness and being in bed are used in the following.
Depending on the extent of your relationship, she might need to reach a comfort level. There cannot be any doubt in her mind that you wish to pleasure her this way. Talk to her; tell her explicitly what you are going to do. Tell her how you desire her, want to pleasure her, to drink her, to kiss her deeply there; whisper all this to her. She wants to hear how you are going to please her, so oblige her.
How soon should you approach her there? There is no rule. Only you and she can decide when to go southward. Personally, I think that there should be wetness before you go there. Kiss her, touch her, taste her skin. Touch her breasts, love her nipples; respond to her. Make your pace the same as her pace. Kiss your way downward, slowly, enjoying all of her. The closer you get, the more her scent will reach you. Reach down, and gently touch her, and enjoy her wetness. Pause and move back up, kissing her, and tell her how much her scent arouses you.