You'd be best off paying to have it done, but if you do want to go ahead with it, really you'll want 2 tanks.
First one to 'dip' it through would be a caustic solution, if you can heat it, so much the better, but even cold works well. This solution will clean the oil, grease, grime and possibly paint from it. Heated, this usually only needs about 10 - 30 minutes.
Wash it very thoroughly before going to the next tank as the caustic is an alkaline solution.
Next up, as mini13 has suggested, an Acid bath. I use Phosphoric Acid for this.This is to remove the rust. On a suggestion. last week I tried a product call Evapo-rust. It did the job but after 1 block the solution had lost it's zing. It was also much slower than the acid. The Phosphoric Acid usually only needs 5 - 10 minutes. To me, this step is very important to allow the cooling system to work properly. Probably 60% or more of run hot / over heating problems I've seen is contributed to or because of a rusted cooling jacket. 1/2" layer of rust has about the same heat transmission capability as 6" of steel plate.
Naturally, all Personal Safety will need to be done too, including gloves suitable for chemicals, an apron and a face shield are all strongly recommended. You should also consider how to dispose of these chemicals before setting out too.
Also, as min13 suggested, pull all the plugs from the oil ways and the freeze plugs, not only to allow these solutions to get in to where they need to but also for inspection prior to assembly.
Coming back to having it done, if you have a few to do, it may pay off to do it yourself, but to do just one block and head it will be far cheaper to have it done.
Just working the block over with a garden hose won't do much.
