pizzadequeso
It looks like you want to do this as quickly as possible from lock screen, right? You could assign a shortcut for this: Settings > Wallpaper and style > Shortcuts.
You can then assign Mute to the left or right shortcut.
Here I have to unlock, volume then tap.
Just a note: You don't need to unlock the screen to mute the device: wake up the screen, press the volume up or volume down button, and then mute using the button above.