Leaves+ adds a new plant, the harvest fern.
The harvest fern gives harvest leaves when broken, which can be used to make leaf blocks that can be dyed and a new food item, the salad.
To make the salad you need 2 harvest leaves, which are obtained from the harvest fern, a bowl, and a sweet berry.
To dye the leaf blocks you put a dye of your choice in the middle, and harvest leaf blocks surrounding it.
You can either craft the harvest fern with a fern in the middle and 4 seeds around it, or trade it with a farmer villager.