Drag and drop (or click the button to add files from your computer) all .sol files used by the target contract you want to verify and the .json file containing the contract's metadata. For example, this .json is created by Truffle in ./build/contracts folder after truffle compile. If your contract has linked libraries you should also drag & drop .json files _for those libraries. Once all files are added, start verification by clicking the _Verify & Publish button.