The Illustrator gradient mesh tool can be addicting and can bring life to otherwise flat images. It is a super easy way to paint objects.
Start with a good photo of a banana.
Lock the layer the banana photo is on and create a new layer to draw on.
Draw a rectangle on the new layer.
CMD click (Mac) or CTRL click (Windows) on the eye icon in the layers palette on the layer the rectangle is on. This makes the fill transparent which makes it easier to adjust the rectangle to fit the banana.
Use the Add Anchor Points Tool and the Direct Selection Tool to add anchor points and move them into position around the banana. Use the least amount possible of anchor points to keep the rectangle as simple as possible. This is important when the gradient mesh is created.
Use the Anchor Point Tool to smooth the anchor points added to the rectangle. Click on a point and drag to create drag handles that can be moved and adjusted with the Direct Selection Tool.
Finish positioning and smoothing all the anchor points.
Use the Gradient Mesh Tool to add the mesh to the banana. Carefully click along the edges to create mesh lines similar to the image below. If you need a mesh line to go in a different direction and simply using the mesh tool is not working, use the Add Anchor Points Tool on mesh lines where you need them and then create mesh lines along those lines.
Color the Banana
Once you have your mesh you can start coloring the banana by using the Direction Selection Tool to select points along the mesh to color. Select the points and then use the Eyedropper Tool to sample color from the banana photo. Switch between showing the fill and no fill by clicking on the eye icon in the layers palette to see how the banana is shaping up for color.