Mix 天草 and water, bring to a boil. Add vinegar, cover (I forgot this last time) and simmer for 30'. Squeeze through a cloth, gathering the liquid. I had some 600 ml left last time and it was a tad too firm for my taste, so I'd aim for about 700 ml. Fridge to set.
Wash peas, then soak in above liquid overnight. In the same liquid, then, bring to a boil and simmer for 5'. Strain, then soak in 400 gr water and 12 gr salt for 15'. (The original recipe omits the 10 gr salt in the overnight soaking, but I found mine only subtly salty, so I'm adding it here). The steam in a cloth on high for 50'.
For the syrup:
For the fruit: I used a dried apricot reconstituted in 4 T sugar and 4 T water. Next time, however, I'd probably poach a real apricot in wine and sugar until it really falls apart. The dried fruit, however, added a nice 名残 touch, so it's not that bad (maybe adding a splash of apricot brandy to give it more oomph next time).