To make this bike rack you will need:
-an old baby bike seat (looking online it seems like most will work for this)
-a small sturdy wire rack (I got mine from an old camping grill but a small microwave oven rack or any sturdy wire rack would work)
-a few zip ties
-some paint if you wanna make it look nice
I apologize I don't have any pictures of the raw materials because I made it a few months ago, but a quick search of google images shows all sorts of baby bike seats that look similar to what I used.
To begin, cannibalize the baby seat to get the metal frame off. This will form the base of your rack. For the rack I used it was as simple as removing a few bolts.
Now is the point where you would want to clean it up and remove rust and dirt from the wire rack and baby seat frame if you want to paint it. I left the baby seat frame but painted the rack so it matched.
Once the paint is dry (assuming you painted it), attach the frame to your bike and adjust it so the wire rack will sit level on it (or any angle you prefer).
Now all you do is use a few zip ties to secure the wire rack on. I ran one through every bolt hole to make sure the rack was secure and wouldn't slide forward or back. I also ended up having to put 2 on the front of the rack as a safety measure. Use as many or as few as you feel you need to make it secure.
Now you are ready to bungee on a milk crate or whatever you want on back. Alternativly, you could probably skip the wire rack base and attack the milk crate directly on, but I wanted to be able to take the crate off to make for easier storage and more versatility.
Step 2: Go Forth and Ride