Since your workout is based on intervals, adding intervals is an important function in the Session Designer. There are a few ways to add intervals described below.
Here is a video that will show you how to add intervals
You add intervals by clicking the 7 color-zone-circles. The colors can be changed in your settings if you prefer another color combination. You can also add power zones or heart rate zones if needed. The intervals are 1 minute by default, but you can edit all intervals by clicking them and activating the toolbox.
When an interval is selected the toolbox for that particular interval appears. At the top of the toolbox, you will see the same color-zone-circles (now smaller) and you can use them to quickly add a new interval prior to the one you are currently editing.
If you move the time-indicator-bar to a certain position on the timeline and add an interval, the length of the interval will be equal to the amount of time between the ending of your last interval and the position of the time-indicator-bar as shown below.