In "The Chaser," how does the old man trick Alan step by step?

The old man doesn't precisely trick Alan; it's perhaps more accurate to say that Alan tricks himself.
When the old man offers Alan a love potion, he tells Alan in no uncertain terms exactly what the potion will do. The concoction will make Alan's love interest want "nothing but solitude and [Alan]," it will make Alan "the sole interest in her life," and it will turn her into the type of wife who "will never allow you to be tired, to sit in a draught, to neglect your food. If you are an hour late, she will be terrified." To Alan, this sounds like true love. But the old man, along with the reader, recognizes the description as that of an overbearing obsessive stalker--not true, healthy love.
However, the old man unequivocally deserves credit for fooling Alan in one respect: the "glove-cleaner," an untraceable and lethal poison. He introduces it to Alan at the beginning of the conversation and reiterates it after Alan is sold on the love potion. The old man wants to make sure that Alan remembers the glove-cleaner, because he knows that Alan will soon realize the toxicity of his relationship--and will swiftly return for an escape from his overbearing lover.