You turn her into a skin

One quick zap with the black "pen" and Katie is a Katie suit that crumples to the floor of the bathroom, the wrapped towel coming off her hollow head. You get a strange erotic thrill as you fit your legs and then your whole body inside her, though your erection is soon replaced by her lubricating pussy and the nipples of your Katie breasts grow hard even before you pop her head over hers and assume her form completely.

You wrap your new body in a towel and your new head in the same one the real Katie had used, then went to get the brown "pen" and zap yourself: the instructions claim that this will give you Katie memories and a masking Katie personality, and you want to know exactly what she did when wearing you.

The zap works perfectly.