First stumbling block;

Hi, I am a brand recent user and still following the documentation step by step so my knowledge is close to zero. I am trying to take one chapter from a commercial film which has me and my parents ( uncompensated of course) for less than a minute. However, following the instructions ( I think) I select chapter 4 and other steps. When I look at the preview I am getting the 5-6th chapters and the image is incredibly pixelated and‏‏‎‏‏‎‏‏‎‏‏‎­"smeary" The steps in the documentation don't seem to precisely match the program. Are there better sources of instruction? Any thoughts on what I have missed?