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.
Step 2: Make the Holes
Remove the lower seat cushion and check the floorpan for mounting locations. The bolts need to be accessible from between the upper and lower seat cushions. My floopan had a handy design stamped into it, marking a good spot to place the bolts, roughly 1/3 away from each other and the sides of the car.
With the upper seat cushion still in place, position the eyebolts so that they are flush against the bottom of the cushion and mark the area that the base touches.
Once the positions are marked, use a punch or drill to make a pair of holes about 1/2" in diameter. While this shouldn't be a problem in most cars, you should check to make sure there are no fuel lines, brake lines, or electrical wires located on the other side of the sheetmetal where you intend to place the hole.
Step 3: Secure the Bolts
Begin by placing the nut, lock washer and cut washer onto the eyebolt, and then place it into the hole. Using the nut, adjust the depth of the eyebolt so that it sits behind the upper cushion. You don't want to be able to see it without pushing on the cushion and you don't want it sticking out at all.
Once you have the depth set, climb under the car to place the remaining nuts and washers onto the opposite end of the eyebolt. Be sure to put the cut washer on first, followed by the lock washer and finally the nut.
Step 4: Modifying Car Seat LATCH Belt
Because the 1/2" eyebolt is significantly thicker than the bars used in factory LATCH systems, the existing latch connectors will not work. Simply slide a carabiner through the loop in the belt. Be sure to only use climbing or safety grade 'biners. I find that this step is a big improvement to the original system, and is more robust in addition to being easier to deal with.
Step 5: Install the Car Seat.
The final step is to install the car seat by hooking the LATCH system into your new retrofitted attachment points.