To make Cotton in Little Alchemy 2, you will need to combine the following items in order to create it: Plant, Pressure, and Wind.
First, combine Plant and Pressure in the Little Alchemy 2 main screen by dragging both items onto the Little Alchemy 2 workspace. This will create a specific type of Plant called a Cotton Plant.
Next, to turn this Cotton Plant into pure Cotton, you will need to add the element of Wind to the Cotton Plant. Drag the Wind element onto the Cotton Plant and click on the resulting item to create Cotton.
You can then use the Cotton you have created to make other items and discover more new elements in Little Alchemy 2. For example, you can combine Cotton and Cloth to create a Doll, or combine it with Record Player to create a Turntable.