inkscape>share>tutorials>making_markers.svg wrote:
Making Markers : A short howto by John Cliff (Simarilius)

Creating a new marker is pretty simple, just follow the steps below:

* Draw the marker you want on the left end of the red line at the top left of this page,
pointing left (unless you want it to point into the line when applied as a marker)

* Apply the "template" marker to any other line in the file (thats what I use the box
around the red line for)

* Open the XML Editor (Shift-Ctrl-X)

* Expand the <defs>, and find the marker with id="template"

* Expand the template marker, and delete its contents

* Rename the marker to something more descriptive by editing its id

* Drag the shape that you created for your marker from the document section
of the XML editor onto the empty marker, so that it becomes the child.

* Apply your new marker to a line to check that it looks how you expected

* Copy the <marker> from here to markers.svg to make the new marker available to all
inkscape sessions. This is easiest to do with a text editor at the moment.

If you make some cool markers, we'd love to see 'em (and include them in markers.svg
if you're willing). Drop a note to the users mailing list, it's always nice to see the end product.

Hope that's helpful,

John