Sorry if this gets asked a lot.
I’d like to build a simple custom shield. How should I go about it? Is there something like a blank shield part (just the PCB and connectors) to which I can add components? Or, are there parts for the connectors?
I would simply use a prototyping shield, but I have a tremor so I find soldering next to impossible.
Welcome aboard, this is actually the first time I have seen this request 
I thought there was an Arduino shield pcb template but I can’t find it. So the first question would be what processor do you want the shield for? Then I’d suggest a google search for the function you want in the shield (to see if you could make it from already available modules for instance.) The header connectors are available in core parts, you drag the default 2 pin version in to any view and use Inspector (the lower right window) to change the number of pins and other parameters to match what you need. A google search for “fritzing part arduino uno shield” turns up this post on how to make a custom shield for the arduino (other CPUs will work the same):
Peter