Alright, you have now thoroughly read and understood all three of the previous posts on how to dial in espresso. (Right?!) Let's summarise it all briefly :
When dialling in espresso, we're looking for balance and a decent mouthfeel, where neither the acidity or the bitterness is overwhelming the other. The three main factors influencing the espresso extraction and therefore flavour are dose, yield, and time. When dialling in a new espresso or adjusting the current one, start with the current recipe and adjust only one factor at a time, and work in small increments. Taste as you go, even shots which you wouldn't expect to taste good. This will help you learn how adjusting each factor changes the flavour and mouthfeel of the espresso shot.
Of course, coffee in general and espresso extraction in particular is an extremely nuanced, complex subject. We've only just scratched the surface here and made some generalisations. This is just to give you a foothold in the world of dialling in, and to equip you to prepare tasty espressos as a baby barista.
Here is a little cheatsheet based off of these few posts on dialling in. Hope it's helpful, let me know if it is, or isn't!
download the pdf here :