Make a sack with the Pen Tool or the Pencil Tool.
Go to Object > Create Gradient Mesh. Use the settings in the picture.
Select the Mesh Tool. Select a anchor point with the Mesh Tool.
When you have selected an anchor point with the Mesh Tool, pick a darker color.
Use the Mesh Tool on more anchor points like in the pictures.
Pick now a light color for the highlights.
Do it with more.