On the next course, take the roll back to the other end and have your helper line it´s bottom edge up with one of the lines on the top of the installed course.  You want a minimum of 2 1/2" of overlap.  There´s usually two lines, one is the minimum, and the other can be used to provide more lap.  You´ll use more felt with the larger overlap, but it probably won´t be enough to fuss about.  Roll it out the same way as you did the first course, but line up on the same line.  Roll it over the edge, cut it off, and pull it tight.  Nail it in place.
Now repeat the process for the first course only you can do it from beneath the course by standing on the installed course.  If it spans a long distance, again put a button near the top edge (down 4") towards the middle.  After you get your 5 pattern, nail the bottom edge every 8".  Nailing this close on the edge will prevent wind from picking it up, even if you have to let it go for a week.
The thing to stress here is, no creases and get as straight as you can.  I´ve seen those guide lines look like an "S" curve due to the method they use to apply them.  Try to make sure the ends are lined up good and if you pull it tight, it shouldn´t matter much if the middle is right on the line.  Just make sure you have the proper lap everywhere.  Also remember that you´re not nailing across the top edge.  Once the next course is laid, you are nailing along its bottom edge through both pieces.  Next up, we´ll deal reaching the end of your roll.