You create the Tritone Substitution by playing the 3 and b7 of a Dominant Seventh. If you move the root a tritone away, the 3 & 7 have reversed. For example: G7 has a 3 (B) and b7 (F). Db7 has a 3 (F) and b7 (Cb). They are reversed. This creates a similar, but different harmony.
Once you understand that, begin practicing the Tritone Sub Exercise (using the Turnaround). To do this, we change all the chords to dominants. For instance: C7, A7, D7, G7 (I VI II V). Do this in all keys.
Now . . . you can change any of the dominant sevenths to it’s Tritone Sub by moving the root a tritone away. For instance: instead of moving from C to A7 . . . . change the A7 to an Eb7 (by moving the root). Therefore you now have two alternatives . . . the regular II and the tritone bIII. You can also change the II chord and the V chord in the same manner.
Do this in ALL keys.