If you want to create a custom shape or symbol, you can use the make-connected-path-stencil scheme function. This function takes the following six arguments, and returns a stencil.
1. A list of lists defining the coordinates of the path. The path automatically starts at the origin point of (0 0). To draw a straight line to the next point, simply indicate the X and Y coordinates of the next point like this: (1 -3) To draw a cubic Bézier curve to the next point, enter a list of six numbers like this: (-6 -2 -6 7 0 5) The first two numbers are the X and Y coordinates of the first control point. The middle two are the X and Y coordinates of the second control point. The last two are the X and Y coordinates of the destination point.
2. A number indicating the thickness of the line.
3. A number that scales the stencil on the X-axis.
4. A number that scales the stencil on the Y-axis.
5. A boolean (#t / #f) indicating whether the path automatically connects back to the origin point (0 0).
6. A boolean (#t / #f) indicating whether the shape created by the path is to be filled.
This snippet demonstrates using stencils as an override for other stencils, and also how they can be used in \markup. The snippet also shows how to use ly:stencil-translate to shift the position of the stencil. This is sometimes needed since the origin point of the path is "hard-coded" to always be (0 0).
Using make-connected-path-stencil has several advantages compared with using ly:make-stencil and path (as shown here: http://lsr.di.unimi.it/LSR/Item?id=623). One is that you do not have to manually specify the width and height of the stencil, since it gets calculated automatically.