I am new at this, but I have made a few turkey fletchings. The longest straightest feathers nearest the outer part of the wings are the primaries and those that are nearer the body of the turkey are the secondaries. They will be a little shorter and broader, and not quite as stiff as the primaries. Both can be used for fletchings. The primaries are a little stiffer and long enough to possibly get two fletchings off of each.
Many others here have lots more experience than me on this subject and can offer more info and correct anything that I may have mis-stated.
Good luck and have fun making your own fletchings.