After going through the Southern Shelf and Three Horns Divide, you next need to find the Borderlands 2 Cult of the Vault Frostburn Cavern symbols. Here is a quick guide on where to find them, as well as a video to support the text.
Hidden Bandit
When you get to the first Bandit settlement in Frostburn Cavern, make your way deeper into the cave. Keep an eye on your left-hand side as there is a hidden exit. Follow the exit all the way up and you should then find the symbol on the floor.

Frozen Symbol
The next symbol is tricky to find. From where you found the previous symbol. Continue through the cavern. Until you get to the entrance of the Firehawk area (where the fire traps start). There should be another path heading to the right-hand side. Follow the path and drop down into the frozen area. Continue following the path and you should come up to a wall in the ice with the symbol.

Borderlands 2 Cult of the Vault Frostburn Cavern
The next area is the Three Horn Divide and Valley. Two big areas, with three symbols each. Click here for the guide.