Create Child Theme for Existing Site

I have created a wordpress site using a theme. I’ve gotten the site mostly to where I want it, but need to make some modifications to the css file to tweak it some more, but realize I should create a child theme first. I know I probably should’ve created the child theme before starting, but it’s too late for that. The changes I have made were through the theme options, menu and such. Is there any way I can create a child theme, after I’ve already made edits as such? Will I be able to keep my theme option settings, menus, etc.?

Please advise. Thanks!