Well, it depends on the e-commerce solution provider. Generally you will need to do two things.
1. Add Bitcoin as a currency.
2. Add a Bitcoin transaction system.
I'd be glad to talk with you about this, just PM me.
I'm working on a Bitcoin back-end for Spreecommerce.http://spreecommerce.com/
It's a personal-time project, so progress is slow. My JSON-RPC library is here:https://github.com/jjeffus/rpcjson
When it's finished you'll be able to set Bitcoin as the default currency for the shopping cart. Then offer Bitcoin as a payment method.