Generally this proof is done by creating a table with columns and rows counted by the integers. By starting at the 1,1 position you can follow a path on the diagonal which will pass through each cell. Not sure how to discribe this as a general function
we have f(1) = (1,1) ; F(2)= (1,2). f(3)=(2,1); f(4)=(3,1); f(5)= (2,2); f(6) = (1,3) etc,
Perhaps you can stare at that for awhile and piece together a usable relationship. Hopr this is of some help.