The main way to start a Race in RaceDay Scoring is to set the Actual Start Time field to the time of day that each Event has started for your Race. This is explained here.
RaceDay Scoring also supports a way to manually trigger start times from your Race Dashboard if you prefer that, which is described below.
From the Dashboard, click Manually Start Events
In the popup, select which Events you are starting by checking the box next to their name:
Then when you are ready to start the Race, click Start Now.
You can start multiple races together by checking multiple Events.
You can note that the timers on the Dashboard will update to reflect your new start times. You can also see in your Scored Event setup that your Actual Start Times have been set to the time of day that you clicked Start Now.