So you went through all the steps in this guide to configure the Custom URL Domain? Its possible you just need to wait for DNS propagation, if you’re not seeing the Okta sign in page hosted on your Custom Domain. Once this is all done, you should be able to navigate directly to that custom domain and be prompted with the widget to login.
After you’ve confirmed its working, you can now start customizing it. I’d recommend reading through this guide that talks through some of the options you have to see if it meets your requirements for how your sign in page should look/behave.