I find that using teaching cards with these diagraphs are very helpful. It gets to the point where students can basically use them like flashcards. It really speeds up the process of understanding the digraphs. I usually start off with these diagraphs first when teaching pre-readers. Not because they are part of the “h brothers”, but from experience I find that students get these the quickest as transitioning from single letter sounds.
Most common words with "th" digraphs - think, than, third, than, and that