This is one of the most basic shuffles that everyone who handles cards should have in their repertoire not only as a flashy flourish but as one of the most well-known and basic shuffling techniques. Once you can master this there are harder variations of this shuffle, such as the one-handed riffle shuffle which requires a lot more practice and patience. Even with this two-handed version some time and dedication maybe required depending on how experienced you are with card handling.
Required Items: A deck of cards
Step 1: Split the deck into around half, you can do this by either prying off around 50% of the deck with your thumb or flicking off half the deck on to your palm to make it look a little more flashy.
Step 2: Hold the two blocks of cards between your thumbs and pinky, ring finger and middle finger with your pointer finger curled in. In this step it maybe difficult to have the cards kept aligned but with practice it shouldn't be a problem.
Pointers: Getting used cards may take a while so play around with cards, either by doing overhand shuffles or dealing.
Step 3: Use your pointer finger as a fulcrum and begin bowing the cards by pulling your thumb up. Make sure that you're not using an excessive amount of force with your thumbs while pulling the cards since this will cause the cards to bend causing a crease.
Step 4: In this position bring the blocks of cards closer to each other until they will interlock when riffled-down.
Step 5: Riffle the cards off your thumbs. This is the most important step, try not to riffle blocks of cards since this will not shuffle the deck. Instead try and be as delicate with your thumbs and allow one card down to fall at a time so that the decks become interwoven. The perfect shuffle is one that has interlocked every single card.
Step 6: Now that the cards are all interwoven curl your hands into each other creating a bridge with the cards, an important point here is to keep your thumbs at the peak of the bridge so that the interwoven cards do not come undone. By adjusting the angle of the bridge you can adjust the speed the cards fall into place. At this point, speed is less the issue but rather if step 4 and 5 were done properly. The less blocks of cards there are, the better the bridge will look.
Step 7: Slowly release tension from your hands and the bridge should collapse in on itself.
Now continue doing a couple other shuffles or go right into dealing and playing your next cards game.
Good Luck and remember practice and patience.