So you already know that 1/2 of your Full Retirement benefit is GREATER than the full benefit which she currently receiving under SSDI and which she will get as her retirement benefit when she gets to be FRA?
I don't believe that they count any extra earnings benefit which you may be receiving if you worked to 70 - so do keep that in mind if it applies when you figure her 1/2 of your benefit.
She should make application around 30 days before she wants the new benefit to start. It should flow smoothly without missing any month. They may need your marriage certificate - but maybe not - I am just more familiar with the widows benefit in info which they need initially.
Going to a local SSA office might be faster but I am assuming that it can be done online also but don't know for sure.
She should register for a My Social Security Account if she hasn't already that way they can send her a notification/verification letter - post it to her account - as soon as it is completed with the benefit described,amount and beginning date. Then she will get another in the mail.