How does the inside sheaves of the primary look? Is there a black ring where the belt is riding? If so, clean that before you do anything else. Scuff the faces of the primary sheaves with a scotch-brite pad and take a brush to the sides of the new belt, as well.
Slipping in a belt is caused by lack of friction between the belt and the clutch. You are trying to maximize that friction by making sure there is no old belt residue on the clutch (the black ring) and scrubbing the new belt to make sure there is no mould release compound still on the sides. The scotch brite pad will rough up the surface and give a better grip, as well.
Adjusting the secondary will help eliminate any bog, and can help some with slippage, but start with the other two things first.
And RJ beat me to it by 4 minutes