Diluting does not change the moles, so c₁V₁ = c₂V₂: to make a litre of 1 mol/L from 12 mol/L hydrochloric acid take 84 mL of the stock and make it up to a litre, adding the acid to the water.
Adding water leaves the same amount of solute in a bigger volume, so the concentration falls in proportion: the stock is 11.98 mol/L, so 1 mol/L needs 1,000 / 11.98 = 83.5 mL. The stock goes into the water, not the other way, because a strong acid or base releases heat as it dilutes and a little water added to it can boil and spit.
It is for dilution with the same solvent, not for mixing two reactive solutions; volumes of two liquids do not always add exactly (ethanol and water, 50 mL and 50 mL, make about 96 mL), so make up to the mark of a flask rather than adding a measured volume of water.
Estimate with the rule, then check it against the calculator that models it properly.
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