The tonearm with cartridge, shell and counterweight is just a simple lever-fulcrum-force system - exactly like a seesaw you probably experienced as a kid.
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Turn/position/move the counterweight until the arm "hovers", meaning it's horizontal (neither pointing up, nor down).
Now hold the counterweight steady with your right hand and with your left hand turn the dial ring (the black plastic part with the numbers) until the zero mark/line is lined up with the line on the tonearm.
Now turn the counterweight (the dial ring automatically turns with it) towards the tonearm pivot until the line of the recommended tracking force (of the stylus you bought) on the dial lines up with the line on the tonearm. Done.
You do the same again with the stylus mounted and set the antiskating knob to the same number. Which stylus did you get?
You might consider buying a digital scale.
And clean the damn thing somewhat!