I have made several infinity dresses and it's really simple.
Basically this is how I pin it.
Take the skirt piece and flip it inside out
Take the folded waist piece and pin it to the inside of the skirt right sides together
now take the straps and pin those to the under side of the waist band (I take the pins out of the 3 layers of fabric and pop the same pin back in but now over 4 layers of fabric but too many pins never hurt anyone) Now as you are holding it and looking at your fabric layers you are looking at the inside of the skirt with a waistband sandwiched between the straps.