When the ball is hit, run to the edge of the bag that is facing second base. You can either anchor at the corner of the base furthest from your glove or you can straddle the bag with your heels at the corners.
Option 1 - Anchor at Corner Furthest from Glove
Slide your foot down the edge of the base then stride out to the ball.
Option 2 - Straddle with Heels on Corners
Pivot to the base then stride out with your glove foot.
Be in an athletic position at the base with some bend in the knees and at the waist. Always see the ball before you stretch and use the whole side of the base to get yourself into the best position to receive the throw. When you stride out to catch the ball, the glove foot and glove should move together with the glove foot landing at nearly the same time as the catch and your face should be back behind the ball. Think "withdraw the head, extend the glove" so that you can "catch the ball with your face". Be firm and control the baseball when you catch it. Use a backhand for all low throws and picks inside of your glove side foot. Only use a forehand for throws outside of that foot.
Setup with your right foot on the corner closest to the pitcher. The toes of your right foot should be in front of the base so that you tag in front of the base after receiving a throw. If the pitcher picks, tag in front of the base and leave it there. Do not reach out and try to tag the runner.
Right Handed First Baseman
Left foot should be off the foul line and in the direction of the pitcher. Turn the foot open a little to make it easier to drop step if the pickoff is thrown behind you.
Left Handed First Baseman
Left foot setup down the line and open a little.
When the pitcher begins his move to the plate, take a step towards second base with your right foot, then shuffle once (replace your feet). It is not about how far you can get off the base. It is more important that your are in a balanced and athletic position with your weight on the balls of your feet as the baseball enters the hitting zone.
First, remember that the primary objective is to get the lead runner out. It can be a tough double play to turn so stay under control and get the lead runner out. If we turn two and get the backside runner, that is a bonus... so, do not rush.
For a left handed first baseman, the play is easy. Just square up to the ball, field it, and make a good throw. For the right handed first baseman, the footwork depends on where the ball is hit as discussed below. Keep your throw to one side of the baserunner. It is a much more difficult play for the shortstop if your throw crosses the base line and it increases the risk of hitting the runner (in which case everyone would be safe). If you are holding the runner, the throwing lane will be to the infield side of the runner. If you are playing behind the runner, the throwing lane will be to the outfield side of the runner. After you throw, get back to the bag for the return throw if you can. The pitcher should be there if you are too far off to get back. It is a more difficult play for the pitcher, so hustle and do your best to get back.
Footwork For a Left Handed First Baseman
Square up to the ball, field it, and make a good throw. First base was made to be played by a lefty.
Footwork For a Right Handed First Baseman
This set of examples is for when you were holding the runner. This will be the majority of the time.
Groundball Right at You
On a groundball hit right at you or a little bit to your left, field the ball and turn counter-clockwise to get your back to the infield. Stay low and replace your feet by taking your right foot to your left foot and then your left foot to the target. Get your feet in line for the throw.
Groundball to Your Left
On a groundball a little further to your right, the foot work is essentially the same but go ahead and touch the base before throwing it to second. Yell "tag!" to remind the shortstop that it is no longer a force play. Keep the throw inside of the runner.
Groundball to Your Right
On a groundball to your right that you cannot get all the way in front of, field the ball as a backhand then close off the rest of the way to get your feet in line for throw to second base.
Playing Behind the Runner
When playing behind the runner, it is easier to get your feet in line for a throw to second than it is when you catch the ball in front of the baseline. On balls hit right at you, turn inside and replace your feet to throw to second.
On balls to your right, field it as a backhand then close off the rest of the way to get your feet in line for throw to second base. On balls hit hard to your left, turn outside (glove side), replace your feet, and plant hard on your right foot to make the throw. If you have to take more than a step or two left to field the ball, it is going to be a difficult play to get the force at second base. Depending on the speed of the baserunner, the better play may be to simply get the force at first base.