Start off with a piece of graph paper. Measure out the thickness of your board, going up and down on the paper. Make a box, going as far as you want the crown to go in the width direction, and the thickness of your board in the middle.
Now you have a box. The width direction represents how far the crown will go into the board. The height, represents the thickness of your board in the middle. (I know this is poorly written and repeats a lot). The top, represents the top of the board.
Now, grab a ruler.
Make a line from the left side of the box, and make it go to the top. After that one, make a line from your first line, going to the deck of the board. From line 2, make a line to the top of the board. Etc etc, as shown below.
Now, you have a general shape of a rail. In actual size. So, you can take actual measurements off of this. Measure the distance from the bottom of the board, to the bottom of line 1. Then measure from the top edge of the board(top left corner), to the top of line 1. Using a
rail band tool, mark along the entire length of the board, both of those measurements. Now you have one line on the side of the board, and one on the deck. Using a surform, trim down until the two lines are barely visible. The cut should be flat from both lines.
Here is an example of the first cut.
Keep doing it. Except you are now going to be measuring from the cut part, to the top of the deck.
Finish them all off. Then make a tool called a "fred". It's pretty much a piece of wood with a 30 deg angle cut along the length. Then you nail a 5.5 inch surform blade to it ( dont knock the nails in too hard, it will crack the blade, and you have to buy a new one)
This is a bad picture but I am not near mine to take a picture. Anyone feel free to put a better one up.
Run that along the bottom rail. But stop about 20" from the tail to create a hard edge.
Now finish up with some drywall paper, blending in all those cuts. Count how many passes you do (all the way from tip to tail!!!consistent pressure!) on each side and then do the same amount on the other side. Keep the edge up so it won't bite.
The
rail band tool has to be square in one end, and you need to somehow attach a moveable marker/pencil/etc on it. I use an adjustable combination square, with two holes drilled in the ruler part, with a pencil wire tied on to it. Make sure the pencil is sturdy.
That's an adjustable combination square. With this, you can just extend the ruler and make the pencil go further out. However, do not go by the markings on the ruler, measure it out yourself.