What engine? A shop is your best bet. There's hot tanking (soaking in a tank for several hours) and hot washing (a big dish washer for an hour). For a quickie, put the manifold in the sun for a couple of hours to heat it up, use Eazy-Off oven cleaner and hose off the crud. You may need more than one aplication. A V8 manifold has cross over passages that get plugged with carbon and may need to be drilled out plus deposits build up in the water passages and need to be wire brushed and/or cut out.