first step for a legacy db

what's the first step for the migration of a legacy db such as:

Thufir,

When you define your models and associations between them, you can
override the defaults that Rails uses. Rails adopts the Convention
over Configuration paradigm, but also includes all the configuration
hooks you'd need.

So, for example, the convention is that the primary key is a field
called "id" and is autogenerated. Instead, you'd tell the fof_feed
model to use "feed_id" as the id.

# class FofFeed < ActiveRecord::Base
# set_primary_key "feed_id"
# end