support that board properly!
You will need an infeed and outfeed support whether it's a roller or a stand, absolutely get something.
Next practice on a 8ft or 10 ft piece of Pine, not your good board. You can make as many passes as you need to feel comfortable. Keep in mind that any variation from the fence or a pause in the cutting process may cause a "hollow" in the cut. Just maintain constant pressure against the fence and constant feeding pressure.... not easy, but possible.
Your saw blade should be set to 45 degrees, and you do not need a jig for this operation. You will want to sight down the cut afterward and see if it's straight. A little work with a block plane may be needed to clean up the ends.
It should all be just fine, but you will need 24 ft of run to make the cuts..... nothing in the way. :smile3:
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)