@_juliannaa you take 10 strings double the length and fold them in half for the base strings. For the 21st base string you take the other end of your leading string. One triangle you make with 11 base strings and the other one you make with 10 base strings. For the 21st base string you can also cut a string double the length, split it in half and use it as one base string. MashaKnots has a video titled "MAKING A LOOP WITH ODD NUMBERS OF THREAD || Friendship Bracelets" that explains it.