I want to police customers traffic into 20mbps.

Ingress policing
Create policy map:

policy-map shape-20
class class-default
police 20M 400000 exceed-action drop
Assign policy map to interface:

interface FastEthernet1/0/2
service-policy input shape-20

Egress policing
Unfortunately, policy-map containing police action cannot be attached to interface in egress direction. So here is how i limit it to 20mbps:

interface FastEthernet1/0/2
srr-queue bandwidth limit 20
srr-queue bandwidth shape 0 0 0 0