Circular product design is widely acknowledged as a foundational step for enterprises to implement and establish viable circular business models. To support this objective, it is vital for industrial enterprises to have an easy-to-apply practical design toolkit that can facilitate the assessment and enhancement of product circularity performance. To address this cross-cutting concern, this article proposes a novel framework called the Design for Product Circularity (DfPC) Toolkit which is an extension of our previous "Closing-Slowing Future-Past" (CSFP) quadrant model, the associated fifteen categories of CE-related products, and the associated product circularity index. The present framework aims to provide comprehensive diagnostics and guidelines to enhance product circularity. The original diagnostic approach presented augments the circularity metrics with a hierarchical calculation principle of a product circularity index, from the elementary circularity indexes of components and parts. The DfPC Toolkit design resulted in the presentation of 15 guidelines for design for product circularity, accompanied by 43 strategies and 225 sub-strategies. The findings of this study contribute to product circularity improvement across various industries by offering recommended alternatives for (re)design. In practice, the present DfPC Toolkit has been deployed on an industrial case study and validated through the proposition of valuable circularity-related improvement targets. The DfPC Toolkit provides a practical approach for industrial enterprises to assess and enhance their product circularity performance, ultimately enabling the transition to circular business models.