You need to move the rear sight right to move the group right or left to move it left. Get yourself a hardwood dowel or brass rod works too as well as plastic rod. Put it on the base of the sight & tap it in the direction you want the groups to move. Elevation works the same way, up is up & down is down.
If by chance you need to adjust the front (not normal) Everything is oposite.
Without knowing of what type of iron sights you have, I am asuming that you have no windage adjustments. In this case, you need to move the front or rear sight, either right or left. On a 336 I have done this by moving the rear sight. I simply tap the dove tail over in the direction needed, with in it's groove. I have used a nylon punch to do this so not to leave any scratches.
If your point of impact is hitting to the left, then your rear sight needs to move to the right. If the front sight is used for the adjustment then you move it to the left, if your point of impact is hitting left.
I have a few single action pistols that I have had to adjust the windage on. I have done this by simply bending the front blade a little.
I am sure someone else here can give you a better way to adjust your windage. But this has always worked for me.
Tap on the base of the sight dovetail , using either a small flat punch and light hammer, maybe a piece of leather or something to not mar the finish. Just a little goes a long way. I usually do this at the range where I can try it out as I'm adjusting .
Suggestion put a pencil mark from the mid point on the top of the barrel and up the sight base so you can see how far it moves. The line will shift. And as previously said a little makes a big difference.