Guess the five-letter word in seven tries. After each guess the tiles change colour: green for the right letter in the right place, gold for the right letter in the wrong place, black for a letter that is not in the word at all. Keys you have not tried yet stay grey.
Þ and ð are letters here, not pairs. Wherever English writes a th that is really one sound, Þorndle writes one tile. So worth is WORÞ and too short to play, while months is MONÞS and fits perfectly — six-letter words are where most of the good ones come from. A word is never spelled with t followed by h: it is one of the two new letters, or it is a word like thyme and pothole that this alphabet cannot write.
Which of the two you need is a matter of sound, not spelling. Say the word: if your vocal cords buzz, it is ð (mother, rather, smooth); if it is a plain hiss, it is þ (thanks, health, throat).
On a physical keyboard, type [ for þ and ] for ð.