It involves using your same email address across the board.
Disney+ launched yesterday with an offer to bundle Hulu and ESPN+ for $12.99 per month. Existing subscribers for both of those services seemed hesitant to accept that offer because they assumed they would receive a duplicate subscription, but that's not the case if you follow these steps.
Visit the official Disney+ bundle sign-up page and use the same email address that you've used for your Hulu and/or ESPN+ account(s). Disney+ will then credit you $5.99 for your existing Hulu account, and/or $4.99 for your existing ESPN+ account.
When I subscribed to that bundle on launch day, it took them about four hours to calculate my billing details. It's also worth noting that you can't subscribe to Disney+ via Hulu as previously reported by other websites.
Disclaimer: As of this writing I'm not affiliated or sponsored by Disney+, Hulu, or ESPN+.