Dee, I just did my 66 moldings two years ago after I finished the car. The bathtub clips you speak of slide into the moldings at the openings near the ends. Hold the piece next to the car and mark on the inside of the piece where the holes are, and slide the clips to the marks. As stated, the end clips have studs that go through the holes in the ends of the doors. The corner pieces on the 'quarter panel extensions' all are studs, and the long quarter trim slides into the end. You may (probably) need to massage the piece a bit to get it right. I understand your apprehension, as any chance I have to screw something up, the longer it takes me to get around to doing it.
I might also add, when I got my 66, it had no side trim on it, and the elongated holes were welded shut. I had to drill new holes and find clips that had a stud instead of the standard ones for the application. AFTER I got everything painted and pushed it outside, I noticed that every hole that was welded shut puckered up above the crease. There is a thread in this section of my resto, if you want to see all I had to go through!
Good luck, take your time and you'll do fine. That old adage about 'measure twice, cut once' is off a bit, we measure about 10 times before we do anything permanent.