Hi,
Once your status changes to READY at https://ceac.state.gov, you can complete the DS-160, create your profile with your Embassy and pay the Visa fee.
Check this website for your country as to how to create the profile - https://www.ustraveldocs.com/.
The UK page doesn't seem straightforward, but I was able to find the K1 information by selecting "Visas" link at the bottom of the UK page, then Immigration Visa, then Finance.