I'm not sure what you mean by not knowing how to set it up. Do you mean you don't have a 4th day when you have time to work out, you don't know what exercises to do, or what?
IMO you should do the program as written the first time around, and then figure out what does and doesn't work for you and then tweak it if needed.
Yeah, IME it's not that hard on the lower back, because you're mixing up the weights.
A word (or several) of caution about this program - because it's autoregulating, a lot of people take it as an opportunity to beat the crap out of themselves. When you're working up to a RM, if you get a rep that's ugly (losing form, and/or grinding it out) stop there. If you keep adding weight you'll constantly be going to failure, which can burn you out.
I know he says to add weight "If you are able to successfully finish the preceding set" but take "successfully" to mean with good form.