I noticed nobody has replied to your request yet so I'm going to give it a shot.
A tri-tone is made up of two notes that are three whole steps apart. So starting from an F note you would go up three whole steps(G, A, B). This puts you on the B making a tri-tone of F and B.
There are six tri tone substitutions:
F7(A-Eb) subs for B7(D#-A)
E7(G#-D) subs for Bb7(D-Ab)
Eb7(G-Db) subs for A7(C#-G)
D7(F#-C) subs for Ab7(C-Gb)
Db7(F-B) subs for G7(B-F)
C7(E-Bb) subs for Gb7(Bb-E)
So if you're playing in the key of C and are playing a 2-5-1 progression(Dmin, G7, C), you could substitute a Db7 for the G7 and play a (Dmin, Db7, C)