As I understand, the Karnaugh map method goes from truth values to a formula, so you are not supposed to symbolically transform your formula into a DNF.
Draw a Karnaugh map 4x4 and label it as shown here. Then for each term in your sum put one mark into corresponding cells. For example, if the top of the table says xy and the columns are marked 00, 01, 11, 10, and if the left of the column says zw and the rows are marked 00, 01, 11, 10, then for the term xz you would put one mark into each of the four cells of the lower-right quarter of the table. You can use "xz" as the mark to keep track of the terms.
If there are n variables total, a term consisting of k variables corresponds to cells of the table. Therefore, you will have 8 + 8 + 4 + 2 + 2 + 1 = 25 marks in total. Finally, since is exclusive disjunction, cells with even number of marks will have truth value F and those with odd number of marks will have T. After that, you can construct a DNF corresponding to this Karnaugh map.