Just last hit creeps with your Q spell. Try to autoattack creeps to put them low so you can kill two of them with one Q. Dodge the ennemy attacks if you can and if he is near and tries to outfarm you, just use your Q spell to last hit one creep and hit him for exemple. (With the new Veigar's passive you can stack ap by hitting ennemies but you don't have to only do that)