The most effective approach for migrating code to Swift is on a per-file basis — that is, one class at a time.
Read this post for best practices to migrate your Objective-C project using Swiftify.
In addition, the following articles may be very helpful for your project migration:
Migrating Your Objective-C Code to Swift [ Apple ]
How to Convert a full Objective-C app to Swift [ Google Developers ]
How We Migrated Our Objective-C Projects to Swift – Step By Step [ Skyscanner ]
Translating your Objective-C project to Swift [ Matt Neuburg, O'Reilly ]
Case Study: Migrating from Objective-C to Swift [ Dominique Stranz, OLX ]
Additionally, here is a very good description for using the Bridging Header file for interoperability between Objective-C and Swift code.