There are 2 basic skills that you must master:

1) Find out the only possible positions of the mines of a number:

For the following case:



There is only one unopened block adjacent to the circled block, so that unopened block must have a mine:




2) When all the mines of a number are found, clear all the remaining blocks:

For the following case:



You have already found the mine of the circled block, so all the remaining blocks must be safe: