In order to transmit and receive on opposite polarisations you will need to assemble the feed so that the receive part of the OMT (which the LNB is attached to) is perpendicular to the ground and the wide face of the waveguide is parallel to the ground.

As shown, the LNB is receiving horizontal polarisation and the BUC is transmitting vertical polarisation.  A rotational polarisation adjustment amount must now be applied, unless the satellite is exactly due north or due south of the location. (text added by forum admin).