Reconfigurable intelligent surface (RIS), capable of dynamically controlling wireless propagation characteristics using reflecting antenna elements, is a promising technology for enhancing signal coverage and improving end-user connectivity in next-generation wireless networks. This paper presents a complete design flow of a modular RIS prototype operating at the n78 frequency band, starting from the simulations to the prototype development and testing. An RIS prototype includes one master and up to sixteen slave blocks, each of which has an identical hardware structure with $8\times 8$ reflecting surface elements and a controller board. The phase shift response of each unit element is controlled with a PIN diode to form a $180^\circ$ phase difference between the ON and OFF states. The measurement experiment using two RIS blocks, horn antennas, and a vector network analyzer showed that the improvement of the received signal power is more than $15$ dB across the n78 frequency band for a given placement.