how do i write a procedure for making a 100mL of 0.5 M solution of sodium phosphate
M = moles/liter of solution. It is important that you remember the difference between ADDING water and MAKING to a final volume of SOLUTION.
You want 0.5 M and you want 100 mL (0.1 L); therefore, you want to weigh out 0.5 x 0.1 = 0.05 mole Na3PO4. You can calculate the grams. That is approximately 164 grams per mole but you need to do it accurately; I just estimated the molar mass. You want grams = mole x molar mass = 0.05 x 164 = about 8 grams.
Place 8 g Na3PO4 into a 100 mL volumetric flask, add a little water, swirl until all of the solid is dissolved, then add water to the mark on the volumetric flask. Note that adding water to make the volume 100 mL is not the same thing as adding 100 mL water.