You just mix it with part of your oils, assuming you are making CP soap or one of it's variations (not MP). However - and someone correct me if I am worng - you may not get the white you desire depending on what oils you use to make your soap. Oils that turn yellow will not become white just because you add titanium dioxide.
Also, it also depends on your fragrance. Choose a non discoloring fragrance. Fragrances containing vanilla will discolor to brown no matter what.