I found Ridi to be the fastest and easiest option, but most sites have guides on how to do it if you search online. You can verify your age by submitting a support ticket along with a redacted copy of a local photo ID (for example, a driver's licence or passport).
The information you should redact is your address, signature, and ID number. The document only needs to show your photo, name, expiry date, and the type of ID. My verification was approved within two days. Most other sites use a similar system (I have Mr Blue and Bom verified too)
If you're using Ridi, go to Profile and select "1:1 Enquiry" from the menu on the left. Follow the prompts to submit an enquiry and choose "Member Account Enquiry". This will bring up a form, and one of the enquiry types is "Overseas Age Verification" (해외거주 성인인증).
Good luck!

is there any way to read the novel version without having to verify my age cause it requires me to have a cellphone number in korea huhuhu