The heart of any decentralized system are the smart contracts which define the transactions and rules which govern the system. Smart contracts are implemented as software modules carefully designed and optimized to execute efficiently and securely. While the logic of a smart contract is often simple, the implementation of real-world production contracts is far from trivial as it requires a thorough understanding of the decentralized execution environment and detailed testing.
To design a decentralized app takes more than just technical expertise. The current generation of blochains require a synergy between technical and business thinking. First and foremost you really need to understand Blockchain in order to design a successful app.