Adding transitions on imovie?? Help!

Ladies, When u add the transitions to your videos on imovie like "fade to black" or "cross dissolve" etc.,....when do u add them? Ok, if I want to show a carousel spin, then after that a fireman for ex. would I put the transition in between the two or...would i stop the recording and film each one sep? This is driving me nuts! I taped a whole dance then tried to add transitions and it wouldnt let me. I could only add where there was a green line. On many of your vids, you go from one pose to the next with a fade out,etc in between. How were u allowed to do that unless Im doing something rong...which I probably am. Thanks...