Regardless of the firearm, there are four fundamentals (or so says the Army): sight picture, steady hold, trigger squeeze, and breathing.
As said, get a good sight picture. Focus on the front sight all the way through the shot.
If off-hand, make sure you are in a proper stance with the rifle supported. Off a bench or sandbags, just resting on your left hand (for a right handed shooter). Cheek weld to the stock in exactly the same place every time. It should be comfortable. Find something for a reference. Butt in the crook of my shoulder and cheek on the rest works for me. Right hand same place on the grip, trigger finger same spot on the tip of the pad.
Squeeze, don’t jerk the trigger. Even more important with a “creepy” trigger. Don’t worry about when the ignition system fires. Just a steady squeeze.
Some inhale and let off half a breath then squeeze. Some exhale fully the take a half breath and squeeze. The point is to relax and do the same thing every time. Every time you breathe, your chest and body moves. You want to fire with the same rhythm each shot.
Instead of a quarter, we used to use a dime on the barrel of our service rifles. If you could dry fire and not drop the dime, you had a steady position and good trigger pull.
Works with flinters. I’ll pick one up randomly, aim at a spot on the wall or leaf out the window, and follow the same dry fire routine. I burn up some flints but flints are cheap.
I’m not a competitive marksman by any stretch. But I like to shoot and hunt. I want to know where my rounds are going to hit.
Once you’ve got yourself tuned up, work on tuning your powder charge, patch, lube, and all combo. I started with Dutch Schultz’s system then found what works for me.
Keep at it! I’m constantly surprised at how well these old fashioned guns can perform.
Paul