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A couple of years ago I received an M-14S for Father's Day.  When I got it, I found the two techniques for zeroing it.  Here they are, hope that they work for you.

I can't remember where I got these two snippets from, so apologizes in advance for not providing the appropriate credit.

The rear sight does most of the work. Here's what you do if you have a 100 meter or yard range:

1. Zero your rifle and adjust your rear sight so that POI is Point of Aim at 100 m (simpler that way)
2. TURN DOWN (forward) the rear aperture and COUNT # of clicks heard/felt on your descent. Write this number down.
3. Flat bladed screw driver needed: Loosen the rear sight elevation pinion screw until it spins freely, confirm this by turning the elevation knob forward and the aperture remains bottomed out.
4. While turning the elevation knob FORWARD, locate and align the 100 m / yd zero index line. Now ROTATE the knob FORWARD the EXACT number of clicks that you recorded in step #2 above. Tighten firmly (not tight as fzck).
5. Rotate rearward, raise your rear aperture all the way up and gently tighten again.
6. Final test. CLOSE your eyes and rotate the elev knob forward to absolute bottom. Now back off the number of clicks your rifle and ammo combination needs to zero at 100. You SHOULD be lined up at the 100 zero.

Here's the zero procedure for 25 meters.

1. Set the windage marks of the receiver and sight base so they are centered.
2. Crank up the sight aperture all the way up. Push down with thumb. If the aperture moves, tighten the screw in the windage knob (right knob) 1/2 turn clockwise and repeat.
3. When sight tension is set, aperture will not move under thumb pressure. Crank the rear aperture all the way down and then back it up 8 to 12 clicks.
4. Sight-in rifle at 25 meters. This should equate to a 250 meter zero.
5. Loosen the elevation knob screw and rotate the knob so the marking is mid way between the 2 and 3 marks (250 meters) and match with scribe mark on left receiver ear. Tighten screw.

Note: If your windage is off with the sight base mark centered on the receiver mark. Your front sight is probably off. loosen the socket head cap screw and move it a little bit at a time to compensate. If it is 'way off, then your barrel is probably not correctly aligned (top spline on barrel not within 90 +/- 1/2 degrees.