ANy tips on how to do tritone substitutions (bII7 for V7) in Hookpad
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Borrow the chord from the mirrored mode. The mirrored mode is found by descending with the same intervals of the scale. Let me give you an example. The pattern of the major scale is WWHWWWH. On a piano keyboard, it’s visually symmetric on D and Ab, so let’s use the D major scale.
Following the major scale pattern ascending, we get D, E, F#, G, A, B, C#, and back to D.
Now following the pattern descending, we get D, C, Bb, A, G, F, Eb, and back to D.
If you play this on a piano keyboard, you’ll see the visual symmetry I talked about. This only occurs with the D and Ab keys, but it should give you an intuitive understanding of what mirror harmony is.
Anyways, so what was that descending pattern we derived? It’s the D Phrygian scale. The 2nd harmonized chord of this scale will give you Eb major which is the bII of the original major scale, the tritone substitution.
To save you some time, here are the mirrors.
Dorian is palindromic, behind the mirror of itself.
Mixolydian 
Aeolian (Minor)
Ionian (Major) Phrygian
Lydian Locrian
Hope this helps! 
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They’ve updated the system recently to include tritone subs and published an article here: The Ultimate Guide to Tritone Substitutions in Popular Music - The Hooktheory Blog