I agree with Mike, with one exception. If the engine is really cruddy, go ahead and use the flush. Only use the quart that you add, idle ten minutes then drain. I would see that the engine is at operating temperature before adding the flush.
If it's not crudded up too bad, I would do hot oil and filter changes about every 500 miles until it is all cleaned out, that's the safest way.
The flushing that you definitely want to stay away from is cranking the engine with nothing but solvent in the crankcase.
After you have the engine clean, then start religiously performing hot, frequent, oil and filter changes.
As Mike said, this is the best way to clean the engine.
Change oil hot and change oil often,