Practice covering first base on balls hit to the right side of the infield. The first baseman sometimes needs to field a grounder, meaning first base is left empty. The pitcher has to scoot from the mound to first base to catch the throw or flip for an out. Simulate that situation by having the pitcher throw a pitch. Then have the coach hit a grounder or roll a ball off first base as it would happen in a game. The pitcher needs to run to first base and then turn about 10 feet away from the base. At that point, he continues running with his glove up at chest level as a target for the first baseman. He needs to touch first with his right foot as he's running.
Back up third base and home on throws from the outfield. A pitcher is the last line of defense from an outfielder's errant throw to the third baseman or catcher. Perform a PFP drill in which the outfielder purposely throws too high or too wide. Have the pitcher back up the play and grab the throw.
Field the ball when it's hit right back to you. With comebackers, the pitcher has no choice but to make the play. He also has to throw to the right base. Practice different scenarios. Have the coach hit a ball to the pitcher with the bases empty. Then, load the bases and have the pitcher throw home for the force out. Also, the coach should hit line drives, sharp grounders and bunts so the pitcher can become comfortable in all situations.