To add a transition, go to scenes list on the left side of the screen and click on the scene you want to add a transition to. Navigate to the right hand side of Synthesia, and turn the Scene transition toggle to on.
Once Scene transition is switched on. You will be presented with a drop down menu and a choice of transition styles. From here, select the style of transition.
To add another transition, click on the scene you wish to add the transition for a repeat the process.