GIS plans - duplicating side panel

I am a "first time builder." In reading over the plans, the description of duplicating the first side panel has me scratching my head a bit. The way I read it, it says to trace around the original panel and cut/plane to the line. That leaves you with a second panel that is necessarily bigger all around. "Plane further if necessary" sounds like an understatement. I'm curious how others have achieved duplicate panels. A cursory search of this forum doesn't return any obvious mention. If I missed something, please point me in that direction. A couple of options that occur to me.

1) mark out first panel, make off-cut, flip off-cut under first panel layout, and cut both sides at once, plane both to line at once.
2) follow directions literally, fasten/clamp sides together, plane second side down to first side.

Plane before or after joining side pieces with butt blocks?

Am I overthinking this?