Step 8: Just do it now!!
read the previous steps. get an efficient speaker (or 2), get an effiicient amp, a battery, and plug them together. done!!
(my article about a simple rear-rack mounted bike party sound system shows an example of the 'do it now' option)
the steps after this show how to make an Active 12V speaker - a speaker with integrated battery-powered amp. This allows slightly improved efficiency, lower weight and a more convenient package, but it is also more complex to build.