Using ax'' + bx' + c as an example.
Set x = x_1 and x' = x_2
Then x_1' = x_2 and x_2' = (c -bx_1')/a

From my previous thread I gathered that I needed to run the runge Kutta algorithm on both of them.

BUT how would I combine them together to form the actual solution?

If you could this step by step what my program needs to do that would be awesome