Step 3: Attach the wheels and front leg
Temporarily clamp on the forks with the wheels installed to figure out where the cart would be balanced so that some weight remains on the front end where it will hook up.
I made a front leg for the cart so it can stand on its own, e.g. if you want to take it into the gocery store (an exercise in social engineering I have not tried yet). The leg has to be in the right place to balance, yet to not interfere with the bike. I initially wanted to attach one of the swivel wheels salvaged from the original undecarriage, but it didn't work very well, so I scrapped the idea and am keeping them wheels for future projects.
Keep in mind you need a strong part of the cart to weld to, so you might be constrained by that. If this is a problem you could weld a different piece of scrap metal to a strong piece of the cart and then to the fork.
Mark the spot, weld on the forks.
Fix welds on the cart that may have come loose in the straightening process.