# How do I prepare a solution that is "2.0 g/L" of "NaCl" in water if I have "2.000 g" of "NaCl"?

Well, you can put $\text{2000 mg}$ or $\text{2.000 g}$ of $\text{NaCl}$ into a $\text{1.00 L}$ volumetric flask... if you have one. In that case, fill it up to say, $\text{0.30 L}$, swish it around until it fully dissolves, and then fill it up to the $\text{1.00 L}$ mark so that there is headroom to swirl the liquid and so that displacement doesn't overfill past the $\text{1.00 L}$ mark.
There's not much else to say, since you apparently already know the mass you need to add. Just make sure the final volume is the volume you fill up to, and don't add the $\text{NaCl}$ after you reach $\text{1.00 L}$.