How to Win Chess Games
I'd like to think that when I win a game it is because I played well. Replaying my won games shows that it was not because of sharp play on my part. It was because my opponent blundered. That's the brutal truth.
At higher levels of play the mistakes are more subtle, but it is a rare won game, even at the GM level, where the losing party has not stumbled. Of course, not all blunders consist in failing to see that the opponent can launch a double attack or similar stratagem on the move. A blunder can be failing to see that a dangerous build up must be defended against vigorously or that a combination is threatened in two or three moves.
So how to make the opponent blunder? Essentially, the way to do it is to put on the pressure, limit their room to manoeuvre, set up threats, lay traps and play good chess. The better placed are your pieces and the worse your opponent's, the more likely that they will blunder.
The other half of the ledger is that the games I have lost were due to blunders on my part. Hence the other piece of wisdom is, "Don't err!". In practice this is impossible, but errors can be minimised. Looked at another way, errors can be maximised in various ways. If I am in a hurry, distracted or too lazy to work out the lines, then error sneaks in with ease. There are myriad other sources of error, such as over-confidence and its opposite, timidity.
Even if it seems my next move is obvious, I pause. Before submitting my move I always make one final check to see what are all the effective moves my opponent can make. By effective I mean moves that impact on my position. This does not prevent me from making errors, but it weeds out most of the obvious ones.
To prevent blunders it is not enough to just look at the opponent's possible response move, because looking only one move ahead is not the way to play good chess. At the bare minimum, all forcing lines, ie continuations that feature captures, capture threats, checks and pawn advances should be worked out until a stable position is reached.