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Preparing a solution of known molarity is a two-step weighing calculation: moles = molarity x volume in litres, then mass = moles x molar mass. For 500 mL of 0.25 M sodium chloride that is 0.125 mol, or 7.305 g of NaCl dissolved and made up to the mark.
Moles required
moles = molarity x volume (L)
Mass to weigh
mass (g) = moles x molar mass
No. Dissolve in less solvent than the target volume, then make up to the graduation mark, because dissolving changes the volume.
The same mass is shown in milligrams as well, which is easier for sub-gram quantities.