Here photos of a jig for cutting the 15-degree scarf joint on neck stock using a chop saw.
The tape is because the fence isn’t quite perpendicular to the base. Two layers of blue masking tape gets it pretty close.
This second lousy photo shows what it looks like just after use. (Well, okay, it’s a recreated scene. This was taken a couple of days after I actually cut the joint.)
And here’s how you set up the cut parts to true up the joint so it all fits properly. Use a plane or a sanding block, your choice.
If you leave the cutoff piece long, like I didn’t, you have more space to clamp and get your tools in place for smoothing the cut surfaces. Clamp the baseboard to a table or something and you can run your plane right off the end and true up the very end of the joint.