First thing to do is to determine the amount in grams of copper sulfate you need for the given concentration. We calculate as follows:
0.125 mol/L (.1 L) = .0125 mol CuSO4
0.125 mol (159.609 g / mol ) = 2.00 g CuSo4
We weigh the calculated amount above. Then, we dissolve in water in a 100-mL of volumetric flask. After dissolving every solute, fill the flask up to the mark.