This is all good advice for shooting smallbore and .223 for target competition. If he wanted to know how to fire a .30-06 standing shooting at a deer, not good advice. So for the benefit of those who want to informally shoot offhand or hunt, I will go over that. I teach both positions (Field and Competition) for 4-H Shooting Sports. (.22 and Air Rifle)
For a right handed shooter, stand with the left foot toward the target, right foot shoulder-width apart pointing at a 45 degree right angle to the right, body bladed at a 45 degree angle. Bend knees and lean the upper body forward; this is the part most shooters get wrong. You don't have to lean way, way forward, like some Trap shooters do, but leaning forward does two things; Tightens the pectoral muscle and makes it a built-in recoil pad, and allows faster recovery from recoil for follow-up shots. The butt is placed in the 'pocket' which is the area of the shoulder where the pectoral muscle sits- too far in, and it sits on the collarbone, too far out and it sits on the joint itself; both are painful with high-power rounds, and while not so with.22's and .223's, it still is not as solid a contact surface. The right arm should be out at an angle that is comfortable; I find that for me, any angle below 45 degrees results in the pectoral not contracting, and a resultant 'mushy' pocket. The left arm should be at a place on the forestock that is comfortable, and should be pushing firmly back into the shoulder. The rest of it 1911 guy posted, from "Focus on the front sight." on down. Good luck, and good shooting.