Public administrations are steadily digitalizing all their procedures. In particular, computational laws – such as taxes and benefits – are increasingly implemented within computers, enabling scalable, automated computations. These computer implementations have four key specificities: they are critical software at the intersection between law and computer science, that will be updated regularly by legal changes, and have a long lifespan, counted in decades. Thus, great care should be taken to avoid any issue in these specific legal implementations. Building upon years of studying and coding computational laws, both in administrations and as new research products, we propose 20 recommendations to ease the development and maintenance of legal implementations. These recommendations aim at being understandable for lawyers and computer scientists alike.