First we draw two shapes, one on top of eachother with the top one being smaller. In this example we will be using a circle. Lower the opacity on both shapes.

Select both shapes.

Go to Object>> Blend >> Blend Options

And this window will pop-up. Choose specified steps and enter a value for the amount of colour block changes you want. For example we will use 12, so there will be 12 shades of colour between the lighter circle and the darker circle.

After clicking OK, go to Objects>>Blend>>Make, and the following will happen.

If you want a smoother blended gradient, choose Smooth Color instead of Specified Steps.

After clicking OK, go to Objects>>Blend>>Make, and the following will happen.

Now you can go to the next tutorial on making symbols.