Cars made after 2002 have a LATCH (Lower Anchors and Tethers for CHildren) system in place from the factory, but there aren't any kits available to retrofit older cars. My car, a 1960 Ford Fairlane 500, didn't even come with seat belts, so I decided to custom fabricate a LATCH system of my own to enable the safe transport of my daughter while avoiding the expense and hassle of getting seatbelts put in.
Step 1: Buy hardware
2 x 1/2" thick, x 6" eye bolt w/nut zinc plated
2 x 1/2" nuts zinc plated
2 x Carabiners (locking ones are probably overkill, but get good quality ones that latch into a hook system when closed)
4 x 1/2" lock washers zinc plated
4 x 1/2" cut washers zinc plated
(image shows disassembled parts on left and assembled version on right. The washers will go on either side of the sheetmetal of the car.