Designing boards
A board is a shared display that every player reads constantly, so its first job is communication, not decoration. Before drawing anything, list what the board must tell players: where pieces can go, what each space does, whose turn it is, how close the game is to ending. Every one of those questions should be answerable at a glance from any seat at the table.
Start with structure, not art. Sketch the board as plain shapes: a track of spaces, a grid, a map of regions. Test whether the spatial relationships work, whether players can reach what they need, whether the track is the right length, whether one area of the board is dead weight. Track length and space count are balance decisions, and you will change them, so pencil and poster board beat anything printed.
Design for the whole table. Text on a board reads upside down for half the players, so favor icons and shapes over words wherever possible. Make important spaces visually distinct by shape or symbol as well as color, both for color-blind players and for low-light game nights. Leave room for the pieces: a space needs to hold the largest pile of components that will ever legally sit on it.
Ask whether you need a single board at all. Modular boards built from tiles add variety and shrink the box. Player boards move personal information in front of each player. Some designs replace the board with cards in a layout. Each option changes manufacturing cost and setup time, so the choice matters beyond aesthetics.
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List the minimum components needed for one playable turn. Cut anything that does not help players make that turn's main decision.
Video tutorials
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How NOT to design a MODERN board game