If the rig is already set up, loosen everything so that you can start from scratch. It is a good idea at this point to lubricate all the turnbuckles. The first job is to set the rake of the mast. If you are putting the mast in the boat for the first time, set the mast so that it leans back a few inches. More rake adds weather helm.
Rake is achieved by moving the butt of the mast forward in the step or the mast aft at the partners.
If you ve been sailing the boat and the helm feels right, leave the rake alone. Be sure the mast is firmly set in the step and solidly chocked at the partners. Use either very hard rubber or hardwood chocks. This product gets poured into the space between the mast and partners and hardens to form a reusable hard rubber chock around the mast. Now, using a halyard or a steel tape measure suspended from the masthead, check to see that it is centered by measuring to the same spot on either side of the boat.