To migrate an iOS app from Jenkins and fastlane to Bitrise, create a new free account on the Bitrise website, add your project, and set up repository access. Review the build configuration and confirm it. Set up Webhooks and trigger your first iOS build. In the Workflow Editor, add predefined steps for managing app dependencies, incrementing version/build number, obtaining certificates and provisioning profiles, building the app, performing UI tests, updating AppStore screenshots and metadata, sending a Slack notification with the build status, and adding Environment variables. Choose between using Bitrise's code signing steps or fastlane match integration. Add new steps to run different lanes in the Fastfile by clicking on the [+] button. Run your iOS CI/CD workflow manually or schedule it based on triggers like GitHub branch changes. The final result allows for seamless building and releasing of applications with no extra effort, and further information can be found in Bitrise's migration guide and comparison with Jenkins.