Holding a runner at first base is similar for both right-handed and left-handed fielders.
When the runner takes their lead, you want to have your right foot just against the inside corner of the bag. Your feet should be almost parallel to the foul line, though the left foot will be slightly forward. You should be in an athletic position and your eyes need to be locked on the pitcher.
Keep your glove up, giving a good target for the pitcher. You should always be ready to react and anticipate a bad throw. Your first priority must be to keep the ball in front of you. A throw that gets by you means extra bases for the base runner.
When the throw comes from the pitcher, apply a quick slap tag. This means dropping the glove to the front of the bag quickly, trying to hit the hand of the diving runner or sometimes a foot if they are standing up when going back to the bag. Make sure to not leave the tag on, but get it down, make contact and bring the glove away quickly. This is a one-handed tag so you should make sure to close the glove well and have a good grip on the ball.
If the pitcher throws to home plate, square your body with your shoulders, hips and feet to home plate and quickly shuffle off to your right in good fielding position, ready to make a play on the ball.
Since you are positioned closer to the hitter, your fielding stance should be a little lower than normal with the glove out in front and down on the ground. This is to make sure something can’t get past, since the ball will be quicker than normal. Make sure you are not in the baseline where the runner can run back into you, which could possibly cause injury.
Picking off a runner can change the momentum of a game in a hurry. Practice this enough and you can shut down the opponent’s running game.