First, here's the easiest situation.
This is what I call a 'corner 1'. The space they're surrounding is a mine! Can you see why? The surrounded space is the only open square- everything else is blank. There is 1 mine adjacent to these 1s, so this square must contain the mine.

This is a 2-3-2. Everything adjacent to the 3 is a mine, because the three squares are the only squares touching the 3. In a game, you would then uncover the squares next to the mines, because the mines next to the 2s have both been found.

More complicated is a 1-2-1. What do you do with this?

The only way the 1's will have 1 adjacent mine apiece is if the square in front of the 2 is safe. So you can uncover that and mark mines in front of the 1's!
Also, several 1-2-1's will often conveniently line up- treat them the same way as you would a single one- the safe square is below the 2.

For this example, you know that the first 1 only touches two squares. So does the second 1. The square next to the third 1 must be safe, because if it was a mine, the first 1 would not be adjacent to any mines.

This is a variation on the Wall strategy above. Can you see why the space next to the 2 is a non-mine? No matter which of the bottom 2 squares contains a mine, this square can be cleared since it touches the second 2.