The Start phase
The Start phase comprises preparatory steps, some of which you won’t need if certain special rules aren’t active. 
If you are playing a scenario that uses the Reserves rules, then apply those rules now.
There are some things that you must declare now if you want to use them:
Some types of special orders, such as Flak, Preparatory Bombardment and Dodge.
Start with the player whose army has the lower Strategy Rating, and then alternate to declare until you both run out of things to declare. Take Leadership tests in the normal way, where the rules require it for you to successfully set the orders you declare.