Number Notation to Arrow Keys.
Two Hands Play Styles
Spread
This is one of, if not THE most common playing styles in FFR and StepMania. The index and middle fingers on both hands are used in this 4-finger style of play. All the arrow keys are usually assigned to different keys on the keyboard with the settings menu. Left middle finger goes with the left arrow, Left index finger goes with the down arrow, Right index goes with the up arrow, Right middle goes with the right arrow.
A lot of players have their setup reassigned to ASKL or QWOP. This setup allow for more room so that the fingers aren't stacked on top of each others.
Advantages : This style provides opportunity for high speed play. Jumpstream, bursts, hands, and quads aren't a problem. Allows you to get better faster because muscle memory develops fairly quickly.
Disadvantages : Learning some patterns initially can prove to be quite challenging.
Split
This style is similar to spread, except you use 3 fingers on one hand and 1 finger on the other. A big disadvantage is when a song becomes particularly dense in the hand where you are using the 3 fingers. Up-down trills become extremely tiresome, and the one-handed trill (an allusion to the spread play style where there is a trill on either up right or down left, thus making it a one handed trill) on your dominant hand is difficult to master. Loss of speed.
Advantages : ease of playing rolls, and big improvement in overall accuracy.
Index
This is one of the more common play styles where you use your two index fingers on the arrow keys (or another key setup similar to the arrow keys).
Advantages : It gives you the "2 feet" feel as well as allowing you to have speed. Fast streams and tricky patterns usually aren't a problem. Trills and running men are very easy.
Disadvantages : The clutter effect - Your hands get in the way of each other trying to operate in such a limited space. Only having 2 fingers on the keys at any time provides some trouble with jumpstream and fast jump patterns as well as hands and quads.
One Hand Play Styles
One Hand 3-Fingers
This style is generally adopted by new players and involves using your index, middle, and ring finger on the arrow keys.
Advantages : Allows new players to get a feel for the game.
Disadvantages : Hitting speedy sections accurately using this method is a bit trickier than the spread style. It's also difficult to gain muscle memory seeing that certain keys might be hit with one finger some of the time and a different finger other times.
One Hand 2-Fingers
This is a very rare play style, but some people use it to simulate your two feet dancing on a dance pad. This style involves using your index and middle finger on the arrow keys.
Advantages : Gives you a challenge while still trying to maintain the challenges of playing with your 2 feet.
Disadvantages : This style can be extremely tricky when attempting some patterns. Only using 2 fingers (especially on 1 hand) translates to not having very much speed, and it's also extremely difficult to hit hands and quads with this style. Keep in mind that keyboard game are not meant to be played on a dance pad.
One Hand 4-Fingers
The placement of your hand with this style is quite tricky. Your thumb goes on the down arrow, index on 4 of the num pad, middle on 8, ring on 6.
Advantages : Allows for greater speed and muscle memory than the other 1-handed play styles. Considerable speed can be gained through practice.
Disadvantages : Very awkward to learn. Thumb/ring finger dexterity is less than that of your index and middle fingers, proving to be troublesome when attempting fast patterns.