To add a question to your scenario, you only have to select the question that preceeds the one you are about to add (when you start this process, you can select "Scenario Start"). Then, click on the question type that you want to add (left panel) and click on the "Add question" button that is on the right of your questions panel.
To start a scenario, you should select first the question that preceeds the one you are about to add. Then, you add a node-question (it is necessarily a Single choice or Drop-down list question). On the panel that is just on the right of your questions, you see then all the potential answers to the node question. Tick the answers that, together, should start a branch and click on "Add question". You can then see the question that is added with the answers you have selected. Start over the process by clicking on other answers, that together, will start another branch and click on "Add question". Now, you can see your 2 branches. You have created a scenario. You can start this process as many times as you like.