In traditional body-art, designs are adjusted to the body as they are applied, enabling creative improvisation and exploration. Conventional design and fabrication methods of epidermal interfaces, however, separate these steps. With BodyStylus we present the first computer-assisted approach for on-body design and fabrication of epidermal interfaces. Inspired by traditional techniques, we propose a hand-held tool that augments freehand inking with digital support: projected in-situ guidance assists creating valid on-body circuits and aesthetic ornaments that align with the human bodyscape, while pro-active switching between inking and non-inking creates error preventing constraints. We contribute BodyStylus’s design rationale and interaction concept along with an interactive prototype that uses self-sintering conductive ink. Results of two focus group explorations showed that guidance was more appreciated by artists, while constraints appeared more useful to engineers, and that working on the body inspired critical reflection on the relationship between bodyscape, interaction, and designs.