Symple is best played on a Go board, using standard black and white Go stones. I’d recommend the full 19×19 Go board (or even larger — more on that later), although for the first few games a 13×13 board or even 9×9 would be a good idea, to get used to the core concepts. Here are the rules:

• Each player chooses a colour, Black or White — White always goes first. Before starting, players should agree on an integer value P, which will affect the scoring at the end of the game.

• Key definition: a group is a set of horizontally and/or vertically adjacent stones (orthogonally adjacent, in other words) of the same colour. A single stone is also a group.

• Making moves: on their turn, a player must do one of the following (players may not pass their turn):
  • Plant: place one stone anywhere on the board that is not adjacent to any stones of the same colour, which creates a new group
  • Grow: add one stone to every possible group of their colour on the board, by placing a stone on a vacant point horizontally or vertically adjacent to that group. If a group has no vacant horizontally or vertically adjacent points, then that group may not be grown.

• Balancing mechanism: once per game, if neither player has yet made a growth move, Black may grow all of their groups and then plant one new group in the same turn.

• Move restrictions: if a group grows and the added stone touches another group, then both groups are considered to have grown (meaning the group the new stone touched now can’t grow this turn). However, two groups may grow in such a way that only the two new stones are now adjacent.

• Scoring: the game ends once the board is completely full. At that point, both players count up the total number of their stones on the board, and the number of separate groups they have. Their final score is the total number of their stones, minus P points for every group they have on the board.

Source: https://drericsilverman.com/2020/04/28/symple-a-game-that-matters/