I share your frustration. The best information we have from the IRS currently is that the digital version of Form 1040X will be available on March 8, 2019. As you have learned, this date is subject to revision.
When the 2018 Form 1040X becomes available, TurboTax will have step-by-step instructions to guide you through the process here: How to amend a return you
already filed
Amended returns cannot be e-filed. Your must
print, sign, and mail Form 1040X, along with a corrected copy of any affected
form or schedule.