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Can somebody tell me where i can find pattern for the square kumihimo disc ? I see/find Pattern for the round disc, can i use those ? How does it work then ? Please help me . Love from Belgium

You have good hands and don't need to buy any. Go into Google and you will have endless ways to make one by our self.
It's basically a square piece with 10 points ("1" -to- "10") up and 10 more ("11" -to- "20")down , then at the left ("A" -to- "J") and ("a" - until- "j")at the right.
The material can be cardboard, plastic, etc. To end you have to make a rectangular hole in the center of the pice, where they exit the woven threads. Good luck.

We have a flat braid tutorial. Some patterns are in there, some are in the comments under the pictures that are uploaded to that tutorial.

O, and no: you can't use the patterns for the round kumihimos to make a flat kumihimo.