How to prepare for big chess tournaments?

^^^

Study a repertoire. Have a response prepared for e4, d4, c4 ( the big 3) when you are playing black. And as white, play e4 or d4 or c4 (only one of them) exclusively and have a response prepared for all the main defenses for it. For example, as an e4 player myself, I play the same openings as a response to the French or Sicilian Defense until I know them like the back of my hand.

Some non-chess advice:

Get plenty of sleep.  Eat lightly--you don't want to play while stuffed.  Work out any logistics--How are you getting to the tournament?  When do you have to leave?  What do you have to bring?--in advance and try to make them bullet-proof so that they don't cause stress.  Learn the time control in advance and set your clock to it.  Bring a snack and drink.

Chess advice:

Unless it is a round-robin where you know your opponents in advance, prepping for a tournament is like cramming for an exam--it's probably better to just have a study plan and keep up with it.  Look at the games from your last tournament, if there was one--what made you lose?  How could you improve that?

If you didn’t study consistently in the months leading up to it, you probably won’t get much from studying the week before. So my advice is to study consistently. If you do that, you won’t have to do last minute cramming.