Plant microbiomes are pivotal for host development, influencing growth, health, fitness, and evolution, and have emerged as promising resources for sustainable agriculture. However, leveraging these microbiomes to improve crop yield and resilience is challenging due to the huge diversity of plant-associated and soil microorganisms and their intricate interactions. Recently, synthetic microbial communities (SynComs) have been exploited as a reductionist approach to harness microbial benefits and to understand multispecies interactions. Additionally, the advanced functionality of SynComs promises to surpass classic single-strain-based biosolutions. Nevertheless, challenges remain in designing customized, robust, and predictable SynComs for agronomic use. Here, we synthesize and discuss the logical and implemented approaches used to design and assemble SynComs, highlighting important principles, challenges, and trends in utilizing SynComs as alternatives to agrochemicals.