On Velocipede there was a tutorial posted by a guy who made tube to tube carbon frames commercially. He showed how he used sanded filler to blend the joints and then over-wrapped them with carbon. Lot's of good detail. I forget the brand.
For a more basic technique, there are lots of pics out there of Calfee's method using carbon tow for building customs like tandems or working with bamboo. The joint is a big knot of tow that is ground smooth after it has cured.
I've always wanted to try doing it with carbon socks, like how fork crowns used to be made.