Remove the porta-filter from the machine & run some water manually through the group head
Dry the filter basket & place 20g of coffee into basket
Ensure coffee is evenly dispersed throughout the basket & then place group onto a flat surface edge & tamp. Tip: Use a cloth of Tamp Mat to protect your bench & portafilta
Carefully insert the portafilter into the machine & place a cup on brewing scales, then tare them off
Start brewing and turn on a timer simultaneously
Stop flow of water once espresso volume reached 40g
This should be achieved in 30 - 35secs.
If your shot finished too early (e.g. 20secs) then you need to make your grind finer to slow the flow of water through your coffee.
If your shot finished too late (e.g. 35secs+) then you need to make your grind more course.
Ensure when you are changing your grind you discard some coffee to ensure the change has come through as most grinders will have some retention of the previous grind size inside the chamber.