Safe Cells strategy
A number counts the mines among its eight neighbours. Use these three patterns to find cells that are certainly safe, easiest first.
A satisfied number
When a number already has as many flags beside it as its count, every other closed cell around it is safe.
All of its mines are accounted for, so none can be hiding in the cells that are left.
Count the mines
When a number has exactly as many closed cells around it as its count, every one of them is a mine.
There is nowhere else for its mines to be. Flag them, then check the numbers beside them: they may now be satisfied.
Two numbers that overlap
When two numbers need the same count of mines and all the closed cells of one lie inside the other's, the cells only the second touches are safe.
The shared cells already hold every mine both numbers need, so none can be hiding in the extra cells.