Originally Posted by: bunmiadefisayodoes this mean that instead of playing the V7, i should just go up three whole tones and use that instead?[/QUOTE]
You can play the tritone sub instead of the V7, or as well as the V7. Right before it or after, then move to the I. Art Tatum was a genius at using this technique.

Originally Posted by: bunmiadefisayoIn what cases can i actually do this, only in the context of a II-V-I progression?[/QUOTE]
No, you can do before any chord you want to prepare for it's arrival. It's really a question of voice leading. Both equator and dvenetian gave you great examples and advice.
[QUOTE=bunmiadefisayo]I was messing around with the keyboard and i was playing something in C#. When moving from the V to the V ...

Can you please explain this again? "Moving from the V to the V" seems like a typo error.
[QUOTE=bunmiadefisayo] ..., my finger hit C# right before i went to the IV so it sounded like another 2-5-1 in the key of F# (which is actually the IV of C#). What is the theory behind this. It sounded really good and what chord do i use as a passing tone btw the V and IV if i do this.

As dvenetian points out the C# is the V of F#, so it could have a solid sounding voice leading preparation effect.

This is the reason that so often the IV chord is preceeded by the I chord made into a dom7 chord. In jazz and blues the I either already is or gets changed to I7 so it really functions as the temporary V7 of the IV chord.

Example: A major (I) --> A7 (I7) (viewed as V7 of IV) --> D (IV)

This technique originated in the voice leading techniques of vast majority of classical music. If you study how the classical masters used the resolution of any 7th chords (especially in a cycle 5 motion) you can see lots of great examples of what dvenetian mentioned in following the voices as they move down through those chords.

There are also a number of explicit uses of Tritone Substitutions in Mozart and Beethoven. Typically, it was closely related to augmented 6th chords.