From Tulum (the village) to Chichen is a relatively straight shot, and it's an easy drive, in my opinion.
The highway west to Chichen ends at Tulum. Depending on where you are in Tulum it could be to the south, north, or even west of where you are staying. Ask the rental car agency if you don't know, but it is very easy to find. Just follow the signs to Coba or Valladolid. Take that highway until it T's at another highway (180) near Chemax. Don't go to Chemax, go west to Valladolid.
Between Chemax and Valladolid you'll hit a bypass road that goes around Valladolid. The bypass is only a short distance past the Cenote Suytun on your left (which is worth a visit -- two cenotes, one underground, the other above ground). The bypass is a very large intersection and goes off to the right.
Stay on the bypass until it rejoins Hwy 180 on the other side of Valladolid. This highway goes directly to the turnoff to Chichen Itza and you won't have to go through Piste.
Have a great trip!