Going left-handed with a baitcaster is not a simple mirror image of right-handed technique. The casting window, button travel, and rod loading all change direction. If you set up a left-handed reel the same way you would set up a right-handed one, the first 50 casts will feel off and you will blame the technique. Here is the setup and release sequence that actually works for left-handed casting.
Buy a left-hand retrieve (LH) reel, not just a reel you flip around. On a LH reel the handle is on the left side and the gear rotation is reversed. This matters because the spool is still wound so the line comes off the front of the spool. If you take a right-hand retrieve reel and just cast with your left hand, the spool rotation direction and the casting window position will fight you on every cast.
On a LH reel, tighten the spool tension knob (often called the anti-reverse or star tension) in the opposite direction from a RH reel. On most LH reels, tightening means turning the knob clockwise (when viewed from the left side of the reel). Test: turn the handle forward and the spool should stop immediately. Then back off the tension by one click for casting.
The most common LH-casting error is copying the right-handed release timing. On a RH cast, the lure swings down and forward, and you release when the lure is at roughly 40 to 50 degrees past vertical. On a LH cast, the swing is mirrored: the lure comes down on the left side and you release when it is 40 to 50 degrees past vertical on the left. This is not obvious in the first few tries because your muscle memory is set to the RH timing.
Your LH downswing arc is slightly shorter than your RH arc because of the way the arm rotates across the body. If you time your release the same way you would on a RH cast, the lure will drop short and to the left. By releasing a few degrees later (more past vertical) on the LH side, you compensate for the shorter arc and get the same forward distance.
On a LH reel, the casting window opens on the left side of the spool. The line release button (the small pad you press to close the window during retrieve) is also on the left side. If you are converting from a RH hand to LH for the first time, the button will feel two millimeters off-center from where your thumb expects it. Give it two weeks of casting and your thumb will adjust.
The handle on a LH reel turns clockwise when viewed from the left. This is the opposite of a RH reel (which turns counterclockwise from the left). When you retrieve, the handle should feel natural within about a week. Do not try to force the retrieval speed. A slightly slower retrieve on a LH reel is more effective than a fast, strained one.
Make sure the anti-reverse is engaged before you start retrieving. On many LH reels the anti-reverse is a small switch or a dial on the side. If it is off, the handle will spin backward every time you pause, which is frustrating and can cause spool tangles.