Before you get started, read Person Account Behaviors to make sure you can live with the limitations. Person Accounts is basically a second-class citizen in terms of features, meaning new features generally take longer to be supported by Person Accounts. For example, Exchange Sync doesn't currently support Person Accounts (future visitors: check Help for features not supported by Person Accounts).
Next, consider enabling this feature in a full sandbox so you can review what happens when you turn this feature on. Since it's permanent, you don't want to get stuck with this feature if it's not for you after all. Salesforce will probably grant you a temporary full sandbox if you ask nicely. Remember to test all your custom code (e.g. triggers, pages, flows) to make sure it doesn't break. You can always refresh the sandbox to undo Person Accounts if you change your mind (obviously, until this feature is turned on in production).
If you decide to go for it, then you'll find it's really straight forward to implement. Person Accounts automatically retain all of the data from both accounts and contacts, so you won't lose any data, nor will you need to merge/update any standard or custom fields. Simply changing the record type is sufficient. As long as you test out the feature beforehand, I don't see any complications with simply turning on the feature.
Finally, make sure you do some research; read blogs about other orgs' experiences, and reasons why they don't use Person Accounts. Even my current employer won't turn the feature on simply because of the limitations.