I'm surprised you're confused. The differences are small. It is the manner in which the behaviour occurs which differentiates the two. Either way, you are showing your superiority.
To be 'condescending' means that someone lowers himself to the same level as the people to whom he is talking in order to show his superiority or the other's stupidity.
Teacher: "No. Pay attention. It's really quite simple. You take the yellow wire and you attach it here and you take the blue wire and attach it there."
Student: "Sorry. I forgot."
Teacher: "Well, I wouldn't expect anyone of your limited intellect to remember which wire should go where."
"You DO know what a book is, don't you?"
"I'm not surprised you fell down. It must be very difficult for you to walk and chew gum at the same time."
Being patronizing is similar except that you are raising yourself above the level of someone else.
"Oh, yes, it's lovely. Mind you, I would never wear an outfit like that."
"I would invite you to my party, but I'm afraid you would have nothing in common with the other guests. You would be bored to death."