Describe the preparation of 2litres of 0.15M CaSO4

How many moles do you want of CaSO4? That's M x L = 0.15 x 2 = ?

Then mols = grams/molar mass. You know mols and molar mass, solve for grams.
Place that many grams of CaSO4 in a 2 L volumetric flask, add some distilled water, swirl until the solid is dissolved, make to the mark on the flask with distilled water, mix thoroughly, stopper, label.
NOTE. CaSO4 isn't all that soluble so if all of it doesn't dissolve then a 0.15 M solution is not possible.