Just as in shooting a small bore target gun, you want to concentrate on that front sight as the gun fires, so you see the smoke and flame leave the barrel before recoil lifts the front sight out of alignment with the shot. That is known as
" FOLLOW THROUGH !"
It does take practice. First, start your practice by just dry firing the gun in your home. Look into a mirror so you can see your eyes closing. Learn to ignore the movement of the cock and frizzen, and then the sparks thrown. Wear glasses for this practice. Then move up to flashing a priming charge with each hammer fall. Do it in the dark of night in the back yard, so that you get the maximum " fireworks" effect of the flash in your face. You will not see this much of the flash in daytime shooting. Then, go to blank charges, so you don't have to deal with recoil. The blank charge will send smoke and flame out the barrel, and with no recoil, much of your good habits worked up withe the .22 target rifle will transfer over. Finally, start shooting at the range, using squib, then mild loads, at short yardage, and only after you are ignoring the flash and movement, load up and shoot off the bench, and then off hand at 50 yds and further.
How to focus on that front sight? Its sounds easier than it is. The way to practice this is to make yourself work harder than you will have to work when shooting normally. Here is How to Practice. Instead of looking at the front sight as a whole unit, mentally draw a line down the center of the front sight, and put that centerline on a thin vertical line, post, string, wire, window edge, corner of a building, etc.( anything that gives you a vertical edge to sight the gun on works). Now go ahead and shoot, but concentrate on keeping that centerline on the vertical target.
When I was very young. my father had my brother and me shoot at toothpicks taped to a piece of cardboard, with our .22 rifles, at only 15 feet. Even from a rest, the guns were heavy enough to make this a challenge, but one we could master. Later, when I was teaching myself to shoot long range pistol and revolvers, and those beer cans at 100 yds, were about 1/3 the width of my front sight, I used the same technique to learn to shoot small groups at long range with open sights on a handgun. You can use the same technique to learn to ignore that flash and movement from your flint lock. Best of wishes, and good shooting. I know you will have fun.