make 800 ml of .25 M sulfuric acid

How many moles do you need? That's M x L = 0.25M x 0.800 L = ?

Then mols H2SO4 = grams H2SO4/molar mass H2SO4. You know moles and molar mass, solve for grams H2SO4.
What you do is weigh out the H2SO4 and add it SLOWLY and CAREFULLY to an 800 mL volumetric flask (I'm not sure they make those things in that capacity) containing about 500 mL H2O. That flask will get VERY hot. Let it cool to room temperature, then add water to the mark and shake thoroughly.