Tips:
1. You don't have to start with something under your toes. Starting on the floor is fine, but once comfortable, elevating the feet to get a stretch through the calf is ideal.
2. The calf muscle involved here, the soleus, can take a lot of weight. Start unweighted and progress slowly, but if the movement feels comfortable and you feel your calf muscles activating evenly on both sides, don't be afraid to load this movement with a heavy plate or dumbbell. The single leg variation might be easier to load if holding a plate or heavy dumbbell.