The rifts are relatively easy to find on the map, actually. You'll spot multiple smaller rifts on the map in purple throughout the game.
You simply need to walk up to them and enter them once you've cleared the region's respective story and dungeon.
We've covered a handful of these in our Side Quests guides, such as:
Stilled Southern Hyrule Field
Stilled Carrot Patch
Stilled Northern Sanctuary
Stilled Hebra Mountain Passage