Actually, it's not that hard. Derm is an advanced program and requires an intern year in internal medicine, surgery, or pediatrics (or Transitional Year). You apply to and interview for both programs during the same interview season (unless you are a reapplicant). Every time you rank an advanced program, a sub-menu pops up for you to make a supplemental rank list of PGY-1 programs that you interviewed at. You can have one Supplemental ROL or multiple ones, but only one list can be attached to an advanced program. You can also rank the PGY-1 programs on your main rank list.
If you match into only a PGY-1 program, you will get "Congratulations, you have matched into a one-year program!"
If you matched into an advanced only program (i.e. Derm, but no PGY-1), then you get "Congratulations, you have matched into an advanced position but not a first year position!"
If you matched to both, you get "Congratulations, you have matched!"