2 things when getting a spinner:
Needs to look decent on the model. For the apprentice you need one about 1/3 the fuselage width (see pictures of Cessna 152, 172, 182 and similar) to give it a sort-of-scale appearance.
Needs to be able to fit the prop. The prop cut outs should be reasonably close to fitting without trimming. Some spinners have guide marks molded in to aid in cutting to fit the prop. If you need to enlarge the cut outs be sure to make them as close to identical as possible in order to minimize unbalancing of the spinner.
For most plastic spinners the backplate goes behind the prop then the cone goes on with 2 to 4 screws after the prop nut is tightened.