so you have a long panel and the lines need to be straight.... this is how i do it. first i get out a long straight edge. line it up where the body line is. (this varies due to body lines.) draw a line with a pen or pencil. that's your guide. lay out your tape line. using the pen mark as your guide. lay down the tape a lil further that the start of the line. pull the tape with a bit of tension on so it stretches but does not pull off at the other end. pull the tape the entire panel and eye ball it to your pen marks and lay it down. this will be done several times to make sure it straight. stand back and look at it from different angles. block sand your panel to the edge of the tape. lay another tape line on the edge of the other tape line. pull the old tape off. block sand the next panel up to the tape line. try to avoid cutting into the tape, ruining your line. pull the tape line off and check your line. this might have to be done several times. once you have it where it's really close. go over the edge cause it will be sharp with 220 or 400 depends at what stage of the block sanding your at. you do this as your doing your mud work as well. pics are not in order but it gives you an idea of what's going on. i didn't plan on making a tech just decided to take pics of the process.