The mitten is knitted in such a way that it starts at the rib and then the mitten body is knitted. The thumb is a so-called invisible thumb. An additional thread is placed where the thumb is going to be. After completing the mitten body, the additional thread is removed and the stiches that form are taken up and the thumb is knitted. It is a fairly simple procedure, don´t be afraid of it.
Always follow the diagram in different direction for each mitten or glove, from left to right for one, and from right to left for the other.