Pick a bottom note for your tritone -- this can be any note on the fretboard. Choose a note that is close to the nut at first until you get used to the distance and sound of the tritone.
Locate the tritone from your bottom note. The tritone will be on the next string (in ascending order E-A-D-G-B-E) one fret higher than the bottom note -- for example, if you play your bottom note on the third fret on the A string, the tritone is on the fourth fret on the D string. Once you have played the tritone, listen for its distinct dissonant quality and remember the position between the two frets that you need to play. Except for the G and B strings, this position will be identical all over the fretboard. For the G and B strings, the interval will be enlarged by two frets. That's because these two strings are tuned a fourth apart, while the other strings are a fifth apart. For example, if your bottom note is on the second fret on the G string, the tritone is on the fifth fret on the B string.
Practice playing the tritone all over the fretboard to become comfortable with the position.