When completed, the sum of all the numbers of 'each' of the horizontal rows, vertical columns and both diagonals will add up to 34 .

<<< Draw the main 16-cell SQUARE . . . . .

then, very lightly draw 2 diagonal lines across the
entire main SQUARE >>>

When inserting the first group of red numbers, start counting from the top left corner -- with the number 1, and on forward to the number 16 -- and put in only those numbers that can be placed into the squares that do __not__ have part of the diagonal lines running through them .

When inserting the second group of red numbers, again start counting from the top left corner -- only now, with the number 16, and on backwards down to 1 -- only now, you will put these numbers only in the spaces that __do__ have part of the diagonal lines running through them .

This is the finished diagram !