Step 1: Tools and Materials
Notebook Computer - found a nice IBM Thinkpad online for $60 with a dead battery.
12V car adapter specific to your notebook - About $10 online.
12V battery - I had an old battery from my garden tractor.
12V battery tender/trickle charger - Already had this but these are $15-30 online.
12V power outlet or cigarette lighter.
Plastic box large enough for your battery, cables and at least a few inches taller. I used a plastic marine battery box, about $6 from W--mart. These are used in boats and RVs so batteries can be moved around. Plastic toolboxes, 5 gallon bucket, anything plastic and large enough can work.
Wire, terminals, electrical solder, battery post clamps (I used hose clamps), cable ties.