You have Drae at level 7 who can do it automatically.
It’s difficult to make a cocktail, as if you make a mistake I think it gives you a bad rating.
You have to choose the color first by pressing the A and E keys for AZERTY or Q and E keys for QWERTY and press S to fill it at the right amount.
Then you have to change colors again and press S until it’s done. Just take your time for the first times, don’t know though if it’s timed but I think not.