for a star at 0 deg declination and 0 hour angle (ie. at meridian):
for each 1 deg error in polar alignment upwards, you will get 0.034 arc-seconds drift (this increases rapidly with increasing hour angle but not much on declination) and 0.004 degrees of field rotation per minute.
for each 1 deg error in polar alignment West, you will get -15.7 arc-seconds drift (this is independent of star's declination) and 0 degrees of field rotation (this will increase if star is at declinations more than ~45deg north or south) per minute.
Why do we need to do this?
a nicely aligned telescope allows the equatorial mount to track the motion of the stars accurately which makes viewing easier and allows for much better quality astrophotos.
although one could use guiding software to correct for alignment errors in long photographs, alignment error will result in field rotation whereby the stars in the outer parts of the image will develop a curved track.
Initial approximate polar alignment:
ensure your finder scope is in alignment with your telescope by locating a bright star in the telescope and then align the finder scope.
aim the equatorial mount at the celestial pole relevant for your hemisphere - in the Northern Hemisphere, aim for Polaris, in the South you have to guess based on the orientation of the Southern Cross (Crux) and Achenar +/- the Pointers (alpha & beta Centauri).
ensure the base of your equatorial mount is LEVEL
If the mount is not level, any adjustments in azimuth or altitude will cause an error in the alignment of the other component.