"Tamping itself is less important than your overall consistency".
What I mean is that you want to keep as many things as you can in your coffee making journey as consistent as possible.
- Use the same amount of beans for each coffee
- use the same amount of extracted water
- use the same time for each extraction
- maintain the same grind setting
- apply the same tamp pressure
- use the same water pressure and water temperature.
Change only one of those things at a time to manipulate your extracted coffee - I usually only change the grind coarseness. Changing more than one thing at a time just creates confusion and affects your ability to repeat that perfected coffee extraction.
So, on to the tamping process. Keep the tamp level with the coffee basket and apply firm and even downward pressure - about 15kg of force is all that is required. If you apply more (or less) pressure, thats fine, do what ever is natural to you, just make it repeatable, because consistency is the key.
Pro Tip #1: get your eye level down to the coffee basket level to be sure the tamp is level with the horizon.
Pro Tip #2: Tamp onto bathroom scales to get a feel for what 15 kilograms of force feels like).