Hi,
the easiest way might be to call WolframAlpha from Mathematica:
WolframAlpha["solve y''=-y", IncludePods -> "DifferentialEquationSolution", AppearanceElements -> {"Pods"},
TimeConstraint -> {20, Automatic, Automatic, Automatic}, PodStates -> {"DifferentialEquationSolution__Step-by-step solution"}]
That also works in plain text:
{WolframAlpha["solve y''=-y", {{"DifferentialEquationSolution", 1}, "Plaintext"}, PodStates -> {"DifferentialEquationSolution__Step-by-step solution"}],
WolframAlpha["solve y''=-y", {{"DifferentialEquationSolution", 2}, "Plaintext"}, PodStates -> {"DifferentialEquationSolution__Step-by-step solution"}]}
Cheers,
Marco