Hi KYMIKE -
I agree with ELDGUELLO's advice, however, notknowingif you're completely new to the sport, let me just add a few details that might be helpful. (This is for a gun that you can look all the way through the barrel.)
Unless you have some sort of bench vise or cleaning stand that you can clamp your gun into, here's an option. Take a cardboard box (open side up) and cut a notch in opposite sides of it to make a "cradle" for your gun. Set it up on a table so you're looking out a window and pick a small "target" about 100 yards away. (Last time I used the end of the peak of my neighbor's roof). Move the box and rifle side to side, and prop it up (front or back as needed) with magzines or whatever works so that you see the "target" right in the center of your bore. Then, being careful to not move the box, move your scope crosshairs (or iron sights) so that they line up on the "target". Recheck averything a couple of times before you remove the gun from the cradle.
This should defintely put you on paper at 25 yards, and you may be surprised at how close you are to being right on.
I know this is very basic stuff, but it may be helpful for any new guys who just started in muzzleloading. As ELDEGUELLO said, it can be used for any type of long gun.