The link to Game Piece Layout (Game Piece Layout) in the Make Layout section of the Game Piece Image Definitions page https://vassalengine.org/doc/latest/ReferenceManual/GamePieceImageDefinitions.html#top of the Reference Manual gives a 404 not found error.
I found the page https://vassalengine.org/doc/latest/ReferenceManual/GamePieceLayouts.html#top by taking the link to the Game Piece Image and then taking the link there for the Game Piece Layout https://vassalengine.org/doc/latest/ReferenceManual/GamePieceLayouts.html#top.