The best way to grip the packets are with three fingers on one side, thumb on the other, and the last finger on the top or bottom (index and pinky respectively). Some people use the index finger of the hand holding the bottom packet to help with lining up the two packets (tip is placed where the two packets meet), while others prefer to have it curled towards the bottom packet to keep it squared. You may also want to experiment with which hand holds which packet, I usually hold the top with my left hand even though I'm right hand dominant, though I have no problems doing it the other way either.
The thing about faro shuffling is really just finding the right angle of the two packets and the right amount of pressure. A methodical approach may help; try starting with a 30 degree angle between the packets and push them together at the nearest corners. Move slowly towards smaller angles (25, 20, 15 and so on) until you find a spot where they start interlacing, then try different amounts of pressure. Another thing that many tutorials don't mention is that at a completed faro the cards will end up at a 0 degree angle, which means that DURING the faro (from when the first few cards start interlacing to when they're all interlaced) the two packets should move from whichever angle they started at to 0. This will keep the pressure constant throughout the interlacing.
Hope this helps! Feel free to post any questions or even images/videos of you trying to do it if you're struggling
PS. As with any flourish, keep in mind that if you're not using a USPCC printed deck or one of a similar standard, the deck may not be able to perform a flourish like this no matter the skill or technique. Just like the coating/finish is crucial to fans, so is the cutting to faros.
