Gradient Mesh - Adobe Illustrator Tutorial

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.