The Krasnopolskiy Method to learning to play blindfold or mental chess is simple and makes training your brain fun. Even kids can try playing this version of mental chess.
First, make sure you have a chess board.
Second, make yourself some chess pieces out of paper, thick paper that you cannot see through is important for this to work. Make sure they fit the squares of your chess board.
Lastly, replace your normal chess pieces with these and play with them face down. At first you may need to check which piece is which often, but challenge yourself to look less and less. If you are playing with someone, you can set the amount of time someone can look at their pieces and lower that number with practice. Also, make sure to limit the amount of illegal moves during the game too!
Have fun and enjoy training your brain using chess!