To dial in an espresso shot basically means that you want to get roughly twice as much liquid as the weight of coffee you used, in about 30 seconds.
For example: if you use 18 grams of coffee ( you will need a good scale too
) you want to get 36 grams of espresso in about 30 seconds. You can use a stop watch or my machine has a timer on it.
Now if you get 40 grams of liquid in 30 seconds, you need finer grind. That's why you need a good grinder that has micro adjustment on it.
If after you made the grind finer and you get 30 grams of liquid you went to far and you need to try again with a little bit coarser setting.
Do that until you get 36g of espresso using 18g of coffee in 30 seconds.
You will have to do the same procedure every time you open a new bag of coffee.
It sounds like a lot of work but it's worth it!!!