stuff some paper towels in the cylinders and water passages. lay a rag or towel in the valley tray. I used a sharp razor blade or you can use a sharp gasket scraper. The razors dull fast so get several. Be careful not to nick the block. If you are just using it on the cast iron, then you can use a light scuff with a scotch brite pad. Do not use on aluminum or you will leave scratches. You can use carb cleaner or acetone and some rags to clean up the heads. Be very careful and take your time. Make sure to get out all of the old gasket and crap from the engine. You want it to be squeaky clean. I also use a shop vac to get out all of the crud and pieces that may have fallen into the valley or cylinders. After you are done make sure not to get any smudges or dirt on the deck of the head.
Autozone also sells aerosol gasket remover, You just spray it on, let it sit, and scrape the old gasket off. Then wipe the head with some carb cleaner.