From what I have read of 'Celtic Golden Dawn' it's a good book, especially compared with some of the other books I have picked up on Celtic magic.
I know that the introductory meditations and rituals in that book are good, and was about to recommend them to you myself since its the first few steps that you seem to be looking for. The book generally follows the papers that an initiate into the golden dawn would receive, but with the focus shifted from Hebrew to Celtic.
There is an adapted version of the LBRP and cross of light in the book, along with the more unique meditation assignments. I think its an especially good read for a beginner because Greer presents a system that is recognisable to magicians of other paths, which means you will have little trouble in adapting his rituals to other forms of ceremonial magic you may find in the future. However, having not read the rest of the book yet I don't know how good the rest of the material is, and it may not be what you are looking for it you wanted a more shamanic approach to Druidism.