This is a really awesome way to spend a couple of hours. The dirt road was the bit I was most worried about but we took a Ford Focus along it with no problems at all. The first bit is definitely the worst - very rutted and quite soft in places - but it gets much better so persevere. It's worth it.
The first bit of the slot canyon is amazing. There was only one ladder we found slightly tricky. The rock arrows make navigation largely very simple. We were unsure just once when we came to a fork while climbing up out of ladder canyon. Take the right-hand path and just follow it to the top.
One more piece of advice, don't wear sandals or you'll be walking on stones all the way back down big painted canyon!