ziploc bag and paper towel is about the simplest way to root the cuttings. but you need to keep an eye on things daily.
first, the paper towel should be wrung out to a point it's just moist. if it's wet.. there is a chance cuttings will mold. second, you should air out the cuttings every day or two. no fresh air will cause mold. third, the room temp or where ever you keep the cuttings should be around 75-80 degrees.
move the cutting as soon as you have roots. leave it in the baggie too long and the things will rot or dry out. i know it shouldn't dry out, it's a closed system. but you are airing it out daily and the paper towel is not holding much water. soon, thing will dry out.