Start basic and very very slow. it's better for them to understand you and to tell you to go faster, than for them to simply not understand you, lose confidence and think it's a bad idea. I have a lot of privates and the key is to understand what they want from the lessons. Some want structure (exam course even) some want interesting conversation, some love grammar. Find out what they want, grade their level and then look for plenty of interesting materials.
Oh, and be ultra happy and smiley. If they like you, it's likely they'll keep up with the lessons, even through the tough stuff.
My 5 pence worth :)