Messenger RNA (mRNA) medicines can express a wide range of proteins in a patient's cells, where they benefit from intracellular expression, endogenous processing and cellular trafficking. mRNA can also be rapidly manufactured, with multiple mRNAs multiplexed into a single lipid nanoparticle (LNP) formulation. These advantages are driving the adoption of mRNA in new medicines, including personalised cancer vaccines, rare disease treatments and in vivo cell therapies. Here, we describe how to build new mRNA medicines. Starting with computational design, we show how mRNA can be engineered for regulated expression and directed toward nuclear localisation, membrane insertion or secretion. We then describe a novel cell-free, isothermal reaction that synthesises high-quality mRNA at scale in a single amplification step, which is formulated with LNPs for precise delivery to target cells and tissues. Finally, we track these mRNA medicines within target cells and evaluate their therapeutic effect. Across each stage, we highlight how cellular biology underpins the lifecycle of an mRNA medicine and informs quantitative design principles. Together, this pipeline has been built into national infrastructure at the BASE Facility, University of Queensland, where we support hundreds of laboratories across Australia to build new mRNA medicines.