First of all buy your Dorans Ring afther that go again to the mid and farm you the first Shoe after that you need the Catalyst the Protector when you have the Protector go and start fights you need 1600 gold for the next Item the Needles Large Rod. Farm with your e-skill all lasthits and after 20-30 min you have the Rod of Ages and then buy your shoes end or buy your shoes end before you buy your Rod of Ages.Now start to buy Rabadons Deathcap after that you need the Rylais Crystal Scepter because Malzahars life is not very much.When you reach the Void Staff your Item build finish. Have fun

Some tipps: Your e-Skill is very OP so if your Virus jump to other Minnions you get Mana.