I think that the easiest way to DRIP is to open an account with a discount broker with fairly low commissions, that has provisions for reinvesting dividends and holding fractional shares.
You certainly can invest directly with many of the companies or their stock transfer agent, but, often the bookkeeping can be a nuisance if you have several accounts.
Have you checked with the DRIP investing board and its FAQ?
http://boards.fool.com/drip-investing-the-basics-100090.aspx...

Bob