These are not endgame principles. “Try not to blunder” obviously applies for every phase of the game, as well as “allways look for checks, captures and threats”. The most important thing in endame is if you have a passed pawn and if you are able to support its way towards queening. “You have to put your king in the center”is not quite correct, you should say activate your king for either supporting your passwed pawn or attacking opponent pawns.