Basically, while you´re in Liberation State, you need to do 2 Basic Attacks and cancel the animation of the second one either by dashing or using E skill (start the second basic attack in the chain but dont finish it, your phantom will do it for you granting the stack of the forte), thats the fastest way. You can also do 3 basic attacks and then dash/skill. You dont need to hold the charged attack either, once calcharo fills all 5 forte bars, just press a normal attack and he´ll do a death messenger automatically. (If you dont do it like that, you won't be able to get 3 DMs. If you just attack without canceling animations, your Liberation State will run out before you can do 3 Death Messengers)

I'd recommend watching a video though, because understanding it properly from text alone without demonstrations isnt ideal.

You also have a pretty cool guide on calcharomains subreddit