I use straight Simple Green (no mix), and a parts-cleaning brush like this...
Get one with a firm, plastic bristles, not the soft-bristled ones (like you find on paint brushes).
1 - Lightly rinse the engine with a garden hose
2 - Let the water drip off until it's just lightly wet
3 - Spray on a liberal amount of Simple Green
4 - Let it sit for 3 or 4 minutes, so it starts working (but be careful, do NOT spray it on a hot engine - a little warm is ok, just not fresh off a ride)
5 - Use the brush, to scrub in the nooks and crannys
6 - Don't let it evaporate before rinsing thoroughly, it can stain if you don't
7 - Repeat as necessary, and follow these instructions. Works like a champ!