Tongue tip between your teeth, same as the first th. Voice ON — this is the th in 'the' and 'that'.
Check yourself: Hand on your throat. Say 'thin', then 'this'. The second one buzzes.
Watch for: /d/ and /z/.
◆ marks a word that does NOT follow today's pattern — Blumenfeld boxes these on the page. Teach it by heart; do not let the class sound it out and lose faith in the rule.
Say the word, have them repeat it, count the sounds on fingers, THEN write. The count is what stops a child writing the first letter and guessing the rest.