I need to add a handling time for personalized items so people do not order something expecting it by a certain date, how can I add this feature to my shipping methods?
In its simplest form you can add the handling time to the shipping methods that are displayed. If you need to do more than this, like change the handling time per product, display dates, calculate blackout periods, etc then you start getting into a complicated extension.
On the frontend there is no capability to add an extra field so you are also getting into modifying on this front. You also would need to persist it so that its seen in the order.
Its definitely do-able, we have a number of extensions that do, but it can turn into major work depending on how deep you need to go. The question is a little vague so I dont think its possible to answer more clearly.