Answer:
Well first you need to be at stage in your relationship where 'touchy-feely' is accepted and warmly welcomed by both yourself and your GF. then you need to establish a comfortable attitude to touching her in various areas. After that, you finally do touch her breast. You can do this by firstly gently brushing your hand across her face, softly, then tracing around the front of the neck near her throat, then lead down to her breast VERY SLOWLY AND CAREFULLY. She will change her body language if she doesn't want you to touch her there yet. You will know because she will pull away from you, tensing up or even squirrel-gripping you and punching you in the mouth :) If you have any other concerns about doing it, talk to her openly and sensitively about if she would like to begin doing that.
Best of luck
-C.G