I assumed I could scan it, but I don't see anything on this topic. Do I have to tape a transparency to the screen and trace it?
You can scan it, and save it as a picture file. In SolidWorks, start a new sketch then go to Tools->Sketch Tools->Sketch Picture.
The help files have a section on Sketch Picture as well.
Scan your image if its in paper format, bring it in as background, stretch/reduce scale against real world reference dimensions, then commence drawing your SW sketch geometry 'on top'. This video shows the basic workflow. I suspect there are additional resources by searching the web.
Solidworks How To Insert a Image - YouTube
This is kind of high speed (because its essentially a teaser ad for the person providing tutorial dvd) but shows the way he uses same process in all three overlapping image dimensions (top, side, front) & works it from there.
SolidWorks Tutorial : How To Model a F-16 in SolidWorks - YouTube
Following up on Jeff's suggestion, there is also an "autotrace" function that can be turned on/off for use with the sketch picture. I would not suggest using it for graphically-confusing sketches, but it may work for simple ones. The autotrace is toggled on/off as an add-in.
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