Install a front-end: Install Spine Toolbox and run IRENA FlexTool as a Spine Toolbox project. This gives you the graphical user interface of Spine Toolbox. https://github.com/Spine-project/Spine-Toolbox. The interface and instructions to it are shown Toolbox interface
Use a browser: IRENA FlexTool can be accessed with a web browser if you have an account for an IRENA FlexTool server. However, no public servers are available at the moment. The browser interface is introduced here.
Local server: It is possible to setup a local server and then use a browser to access that server. See https://github.com/irena-flextool/flextool-web-interface