To create a simulation workspace, navigate to list of workspaces by selecting "Simulating" in the main menu on the left. You will see list of all simulation workspaces you have access to. To create a new workspace, click the round plus button in the top right corner. You will have to give your new workspace a name and select the model and scenario to base the workspace on. The selected scenario defines the data baseline that will be used as the basis for all simulations you create in your new workspace. After clicking "OK", your new workspace will appear in the list of workspaces. Click on it once to select it and you will see its properties in the inspector on the right. You can change and adjust all properties, most importantly the simulation horizon. This setting defines the time interval that will be used for all your simulations.
Open your workspace by clicking on its name. You will find it empty without any charts or assumptions and one default simulation scenario. You can continue by populating your workspace with new tabs, charts, assumptions and scenarios.