It's kind of a big subject, though less so if you already have experience with "long range" percussion rifles. (If you don't, I'd start here:
http://www.researchpress.co.uk/index.php/marksmanship/lrml/pedersoli-gibbs )
The Whitworth itself will follow most of the advice given in that article. The hex rifling does present something of a unique circumstance, but most of the folks who are really making them work well are still using "round" bullets. These should be about .0005 smaller than the bore, with the idea that they will be run down to the breech with just the weight of the steel loading rod. I believe a wad of either vegetable fiber or cloth is always used, and apparently the powder charge should be 70 to 100 grains, depending upon distance. Swiss 1.5g is the nearly universal choice.
Hexagonal bullets are the master class, historically correct but not nearly as common as cylindrical bullets. I know of no sources for pre-made bullets. Pedersoli occasionally offers a mold, but it apparently is of poor quality. Better quality molds are available here and there, from custom makers, at enormous cost. One example:
https://www.castbulletengineering.c...uct/3721-kal-hexagonal-whitworth-bullet-mould.
The best source of info on the rifle, so far as I know, is gunwriter Ross Seyfried, who has explored the topic thoroughly. I am not aware of any online source for relevant articles, though. A determined researcher may do well to contact him directly.
Beyond that, Research Press has a brief article at
http://www.researchpress.co.uk/index.php/research/hexbore/loading-the-whitworth and "capandball.com" has an interesting but limited YouTube video here:
.