There are often situations where you need to trim a group of strips tofollow a line, such as a chine or keel line. You could start by eye-balling the taper on each strip and hoping when you have finished installing them all that you have a smooth, fair line or, you could install a temporary strip then fit each strip one-at-a-time against that strip, but the easiest is to start by not doing any special fitting.
Install all the strips in the area, with them over-hanging the desired line. Then draw the line where you want to trim, cut just above that line with a saw and then trim down to the line with a plane.
The first trick is to establish a smooth, fair line. Some plans will include reference lines on the form patterns to help define where the line is. For example if you are trimming to a keel line, the plans may include a center line on the forms. Otherwise you can just identify an angle in the shape, such as a chine that you will be working on.
You will need some way of transferring the point on the forms to the outside of the installed strip. For this I make a little hook-shaped marker gauge. The marking tip of the gauge on the outside lines up with a straight edge on the inside. By aligning this edge with the reference marks on the forms, the location of the line can be transferred to the outside of the strips.
I start by clamping a guide batten (just a scrap strip) to the outside of the boat in the approximate location of the line I will cut. I then use the marker gauge to precisely locate the batten.
Once the batten is in place, I mark the line on the strips to be trimmed. I use a pen that leaves a bold, visible line so I can't use the excuse that I didn't see the line if I mess up the cutting.
Pull the batten off and use a fine saw to trim the strips. Cut slightly above the line, leaving the line. Note the angle of your saw as you cut. You don't want to cut below the line on the inside of the boat as this can create a gap on the inside.
The saw will leave a rough edge that needs to be cleaned up. You could trim down to the line using a chisel. This takes a steady hand and you need to pay attention to the grain direction so you don't pare off too much. I have found it easier to use a "Rabbet Block Plane". This kind of plane has the blade going right out to the edge. This allows you to trim right up to the forms. It can be a little hard to hold without rapping your knuckles on the forms, but it does a good job of trimming and fairing the strip edges in a controlled way.
With a feature line you may want to run the a strip along the newly trimmed edge, or you may want to have the ends of the strips come in and meet this line like you may on a keel line.
There are lots of situation where you may want to create an edge along an area of strips. Feature lines where there is a knuckle or sudden angle between one area and another are fairly common. There are other ways to deal with this situation but the method outlined above works well.