Assuming a linear polarisation system, you alter the difference between the received co and cross polarisation signals by carefully rotating the feed assembly.
If the wanted signal is vertical polarisation and your feed is rotated so that the LNB input waveguide has its broad flat faces on top and underneath you will have optimum vertical polarisation reception and reception of the unwanted horizontal polarisation interference will be about -30 dB down. You need rotation accuracy of about +/- 1 deg to get this, as the cross-pol null is very sharp and narrow. Make sure you are under the feed arm and not obstructing the beam to the satellite when you make the adjustment. An inclinometer placed sidways across the LNB/BUC is one way of setting the polarisation angle as the scale is often too small to read with sufficient accuracy.
Adjustment by measurement while receiving is difficult. One way is to degrade the quality to some exact degraded quality level at either side and then carefully halve the distance between the two points.
As you misadjust the polarisation angle, the cross-pol level comes up very quickly, while the wanted co-pol signal reduces only slowly. In the case of 45 deg error your wanted signal will have decreased by only -3 dB but the cross pol interference will have increased from -30 to -3 dB. Since the interference is now the same level as the wanted signal you will see plenty of powerful signal but your modem will not decode the downlink at all. Quality = zero !
Here are two images: Two alternative vertical polarisation starting positions (prior to adjustment) and a close up image of the LNB waveguide orientation when set to vertical polarisation starting position. Note it does not matter if the LNB dipole pin is at the top or the bottom, it is still vertical.
Best regards, Eric.