what could make the task of api developments easier for customers?

Hello folks. For some context, I work for a startup and even though is not common, sometimes customer build their own integration to send data to our endpoint, but developers always struggle with our rest api, what do you consider more helpful to ease this task? , normally the only think we provide is a guide with the fields description, and a schema with the structure of how we expect the data to be. Thanks for your input.