Consult with an immigration attorney to determine the requirements.

Gather documentation of your relationship and communication.

Consider seeking guidance on waiving the in-person meeting requirement.

As a US citizen, sponsoring your fiance shouldn't be a problem. However, that you haven't met in person is a big deal and can lead to denial.

I believe you must have met in person at least once and keep records of the meeting. Video chats and online meetings don't count. You may speak with an attorney if there are ways you can prove your reasons why you haven't met and still get the sponsorship approved.