Images used here are from the Beta, however it should be easy to follow.
Upon starting the game, you will come across the above screen. Tap anywhere on the screen to proceed.
The prologue will start along with the opening movie. During the prologue, you will choose your nickname and the name of the story protagonist, which is set to Anzu by default. These can be changed later in settings.
You will then be prompted to choose one of the ten idol units. Your starter card will be a randomly chosen member from that unit.
In the following example, Subaru is the randomly chosen card after the player chooses Trickstar. If you want a different card, you can press "once more!" to try again as many times as you want.
With your starter card ready, move on to Lesson mode.
In lesson mode, you can level up your cards, as well as your player rank. You will also obtain Jewels and Producer points, which are needed for leveling up and obtaining new cards (using the gacha system, i.e. Scouting).
There are five locations you can choose in Lesson mode. In the example below, there is the rooftop, soundproof lesson room, dance room, school gates, and infirmary. Your "Trust level" with the idol will also increase, indicated by the bar underneath each character. This affects the amount of EXP the card will gain at the end of the lesson.
In Lesson mode, you can also trigger mini, normal, special, and intimate events with the idol. Depending on your dialogue choices, you can obtain Jewels, and the ability level of your idol may also increase.
During turns 11 and 1 (indicated by the number at the top left of the screen), the "Dream Idol Festival (Battle)" will take place. Your idols will battle those of a rival unit in three aspects: Dance, Vocal, and Performance.
In this battle, each of the five members of your unit will compete with a member of the rival unit. You'll win the overall challenge by winning 3 out of 5 individual battles. The theme of each battle is predetermined (Da/Vo/Pf), but you can choose which member of your unit to participate in a particular battle. Thus, you can easily win by choosing the member with the stronger stats. Note that you can choose a card only once for the whole battle.
In the Story mode, you can get to know the characters via both the Main Story and Sub Stories. Further chapters will be unveiled as your player rank increases, so do your best in Lesson mode to increase your rank!
Furthermore, Ensemble Stars has a feature called "Idol Road," where you can use Jewels to increase the stats of the idol according to your taste. With Idol Road, you can also upgrade skills and obtain special stories and outfits for the idol.
At the end of Idol Road is a pink box which says "Talents blooming". Upon reaching this box, your card will transform, and the idol's stats will also greatly increase. Do your best to evolve your cards to this final form!