make sure the engine is not hot, kinda warm is fine, not hot. I put hose nozzle on 'shower,' gently and thoroughly wet the engine above and below, spray liberally with whatever doesn't harm or discolor aluminum (simple green? purple power? read label), let sit for a few minutes, don't let it dry, thoroughly hose down. I'm not looking for a show quality engine bay, just one that's cleaner than before.
I do NOT use high pressure. If needed I scrub a bit, but high pressure can drive water past seals into places it should not be.