Quick note: IRL the use of ATS during go-around is technically prohibited (AFM limitation). The expanded language states 'pilots must manually set thrust to override ATS. ATS may be used to trim final N1 once manually set.
Advance thrust levers to the predetermined go around N1 setting (this target should have already been set to TO) while simultaneously pressing TOGA and ATS DISC.
Note: ATS must be disengaged as thrust levers are advanced. If ATS is NOT disengaged and the pilot overrides the ATS as the levers are advanced, the ATS may advance to the forward stop - triggering an exceedance.
Flight Spoilers (if extended) RETRACT
Rotate smoothly at a speed not less than Vref towards command bars
Set pitch attitude to achieve a speed not less than V2+10 as the flaps are retracted in next step to 20
Flaps 20
When climb is positive - Gear UP (there is a note that if a turn is required in MAP, delay this turn until gear is fully retracted)
Retract flaps 'on schedule' (At a safe altitude not less than 400 AGL and speed not less than Vfto-5, flaps UP)
Thrust Reversers OFF
Continue normal climb procedures
These are from the FCOM. This should be practiced and accomplished as a combined set of actions. You commit to the GA pressing TOGA, and then in a series of coordinated steps you press ATS DISC and advance the thrust levers as you're pitching up into command bars. You're verifying the spoilers are retracted and calling for Flaps 20. Things happen quick - you're going to see positive rate pretty fast and you'll ask for gear up. You'll be fine tuning your N1 manually to the TO thrust target. If you were flying green needles on the approach you'll also be asking for your Nav source to be switched back to the FMS (or doing it yourself - I have a button mapped for this) so you'll have the appropriate lateral mode available to fly the MAP. You'll then ask for (or select) NAV and VS. (If you prefer you can press sync to select PITCH, or select FLC but see note below) As you're climbing away you'll be asking for or selecting "AP engage" and setting the speed target to a procedurally appropriate speed - typically the same as a normal climb profile, 190-200 knots. At this point you can engage the ATS as part of the 'normal climb procedures step.
Note, Selection of FLC mode during GA is prohibited unless the altitude pre-select is set higher than current altitude when ATS is engaged. This is small gotcha that can bite you on a GA if you don't sequence things correctly.