Use a crochet hook G/4 mm for size S/M and a 3 mm hook for XS. Use organic cotton yarn or mercerized cotton 2 strand/or other appropriate yarn for a G hook.
For the back
R1 V stich : 1 dc in third chain from the hook, chain 1, dc in the same chain, repeat.
R2: chain 3 dc in the same dc from previous round, (3 dc in the next chain 1 from previous round)* Repeat *
repeat R1 and 2 till you reach the length you need to the sleeves. Decrease 10 stiches in the right and 10 in the left. Then follow the diagram. (It’s from a Chinese or Japanese magazine, i hope i don’t offend anyone.)