Choose the compiler version (according to your contract)
Choose the open source license
If you select Single file as Compiler type, then you can use a library like "sol-merger" to flatten your contract. It creates a single file that contains all the imports.
Make sure there is only one line of '// SPDX-License-Identifier'. If there are multiple lines of this, remove them except the first one.
6. Choose Optimization Yes (if you deployed your contract using Hardhat)
7. Paste the source code
8. If the contract has a constructor then you need to create the ABI code for the values passed to the constructor. Use https://abi.hashex.org/. Copy the result and paste it to the Argument (Constructor) section.
9. Tick the box to indicate that you are not a robot.