To use CF, you just add the domain in CF, it will try to copy the needed records. If that succeeds, you can just change the name servers at your registrar to what they give you and it will work.
If you want to configure manually in cloudflare, you just set the A and AAAA records for the base domain to the IPs shown in Plesk, and create a CNAME for www that’s pointed to your domain.
If you need email to work, there’s additional records needed as well (an MX, and 2 TXT records for SPF and DKIM).