I am using a 10 mm crochet hook and making half double crochet stitches
10 mm Häkelnadel und halbe Stäbchen.
The inner green circle is indeed a T-shirt -
Then I became clever and joined the leftover piece of fabric
with zig-zag stitches and cut it into strips afterwards.
Which is a much faster method
At present I ran out of strips and have to search for more old tees -