You use the equation for the derivative at the initial point to step the solution forward by a half step. Now you use the solution at the half step to calculate the "typical" value of the derivative over the whole step and use this to step the solution forward a whole step from the initial point.

So in this case we have , and we suppose we have the solution at (the initial conditions give us a starting point). We step forward a half step:

which gives us out "typical derivative on the step of .

Finally we do the step:

CB