How to properly configure Cloudflare to work with our SSL certificate?

Question

cloudflare

Hey!

Guys, I'm a complete noob in certificates and their settings, and I ask you to help me figure out how to connect the site to the https://www.cloudflare.com/ service . We already have a DomainSSL SSL certificate provided by the hoster - he installed and configured it, we just pressed the buy button and that's it. The site is working, the padlock icon in the browser is green.
Now the main goal is to filter traffic from DDOS attacks and bots using the Cloudflare service, and I have concerns with this.

Tell me how to connect Cloudflare correctly so that when updating NS records from the domain to the NS records of the Cloudflare service, our site does not fall off due to a certificate mismatch or incorrect configuration of the SSL certificate and its connection in conjunction with Cloudflare? The accessibility of the site is important for SEO, so as not to lose positions, which is why I worry, if the site were new, it would not be scary.

I signed up, added a site to Cloudflare and it found all the DNS records. Next, I need to change the NS servers of the domain to those provided by Cloudflare. And what about the SSL certificate we have, what actions need to be taken in order for our current certificate to work correctly?
1. As I understand it, we don’t need an SSL certificate from Cloudflare itself, because we have our own, right?
2. Do I need to submit something to Cloudflare related to our certificate?
3. What settings need to be made?
4. The minimum tariff, as I understand it, is PRO?

Thank you!