How would you prepare 350mL of 5% (m/v) NaCl solution?

5% (m/v) is 5 grams of a substance dissolved in 100 mL of solvent.

First of all, it's very hard to find a 350 mL volumetric flask (at least I've never seen one in my laboratory), so one would probably prepare a 500 mL volumetric flask full and then pipette 350 mL from there. Secondly, it's difficult to weigh out exactly 5 grams of material, but I won't get into the practical problem of doing that.

For 100 mL of 5% w/v, you want 5.000 grams per 100 mL of water. For 500 mL, you need 25.000 grams.
Weigh out 25 grams of salt and pour it into a 500 mL volumetric flask and fill the flask with water to the mark. As a practical matter, you would use water to wash off the weighing paper to make sure that you got all of the salt that was weighed out.

If, by some miracle, you had a 350 mL volumetric flask, you would weigh out exactly 17.500 g of NaCl, add that to the volumetric flask and fill with water to the mark.