NURIKABE
Pick squares and turn them black. You must create groups of white cells whose size must match the numbers on the board. No black cells must form a block of four in a 2 by 2 group and all black 'walls' must join up.

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Pick an example here

1 - look for 1s  
2 - numbers 1 cell apart 
3 - forced cells  
4 - reach of nos 2  
5 - unreachable cells  
6 - orphaned walls  
7 - fourth corner  
8 - number bubble  

Step-by-step will take you through the main strategies. It may not complete the harder Nurikabes since these require a broader overview of the remaining clues.