As a left armer you really only need your stock ball and leg cutter IMO. Your natural angle should be enough to trouble most batsmen. Use the crease as much as you can and don't be afraid of going around the wicket. Being 1st change, the ball will assist you in getting the ball to swing into the right handers. This is where you should go around the wicket IMO because you want to get the ball swinging late and just enough to hit the pads, the stumps of even the inside edge onto the stumps. The seam should still be prominent, so from around the wicket as close to the stumps as you can, pitch it up on middle and off with a slightly angled seam, about 60 degrees away from the batsman. This will draw in the drive, however the angled seam may get the ball to dart off the pitch and get the outside edge. As far as short balls go, around the wicket is good here too because as long as you don't spray it too wildly you should be safe from the hook and with shallow 3rd man you can dare them to try a lofted cut.
As 1st change the seam may not make a loud 'siffing' noise when you do the cutter. My brother bowls an awesome left arm leg cutter from around the stumps. He'll bowl some safe inswingers to the right hander, not looking for a wicket but not giving away runs. Then he'll bowl the cutter which is the wicket ball. The inswingers will close their stance up then the off cutter will draw the bat away from the body which can get the edge or a forced lofted drive.
For bowling over the stumps to a right hander, while he isn't the best bowler to use as a coaching example, check out Midge Johnson's angle. When he is on song he is releasing the ball almost directly on top of the stumps. This allows the ball to still go across a right hand bastman, but the line holds up enough to entice a drive. What you need to watch is giving away your angle too early, because they will start cutting and square driving with little risk. The idea is to have them worried about their stumps, pads and fielders with the same ball. This is the advantage of a leftie. As a left armer, as soon as the batsman is only worried about the field you are going to get runs scored against you due to the angle and no threat to the stumps or the pads.
There are big differences IMO between a left hand batsman facing a right arm bowler and a right hand batsman facing a left arm bowler. While they are more or less mirror images, the majority of right hand batsman will have very little experience in facing left-armers, yet left arm batsman have spent their whole lives dealing with the right arm bowlers. Use this to your advantage.
And last but not least, work on following through and getting your head up early. With your natural angle and swing into the right handers there will always be the opportunity to get a leading edge for a c&b.
TLDR. As a leftie, get the batsman worried about his pads, stumps and fielders. Once his stumps and pads are safe, you will probably start going for runs.