a.The insoluble base is cupric oxide [sometimes called copper(ii) oxide.] Formula CuO.
CuO is a black powder insoluble in water.
b.To make a solution of copper(ii) sulfate you add an excess* of copper(ii) oxide to dilute sulfuric acid. Heating briefly to boiling point will speed up this reaction. The surplus copper(ii) oxide will not dissolve, so after cooling you must filter the mixture. This will provide a blue solution of copper(ii) sulfate.
CuO + H2SO4 → CuSO4 + H2O
black powder + clear liquid → blue liquid
c.If you leave the solution in an open wide-necked dish the surplus water will evaporate off (may take several days). When the solution reaches a certain concentration (actually about 320g/litre) the copper(ii) sulfate will begin to crystallize out and should form large blue crystals.
[You could speed up the formation of the crystals by heating the solution to evaporate some of the water but the crystals won't look so nice ?].
*"excess" means enough to make sure that there is a surplus which does not dissolve. This means that all the sulphuric acid has been used up.