Step1: Make a selection elipse

 

Step 2: Transform selection to fit your perspective Step

3 Paint bucket black (layer 1)

Step 4: Airbrush hightlight

Step 5: Transform selection to reduce size, create new layer (layer 2) and repeat steps 3 and 4.

Step 6: Create new layer (layer3), paint bucket white.

Step 7: Guassian blur layer 2.

Step 8: Move layer 2 outward to create the look of depth.

Step 9: Add tread.

Tutorial done by Jamie Seymour for Speedanddesign.net