Full mode ssl not working i got error 525

i use cloudflare flexible mode ssl and enerything is fine but when i switch to full mode i got [525 errors] in my website when i searched in the support To avoid [525 errors], before enabling Full SSL option, configure your origin web server to allow HTTPS connections on port 443 and present either a self-signed SSL certificate, a Cloudflare Origin CA certificate, or a valid certificate purchased from a Certificate Authority.

i got the answer from (It is showing error 525 ssl handshaked failed)

but i m beginner i didnt understand how to do it can you show the exact step plz.
PS:
my hosting is infinityfree
pote 443 is working fine

We can’t show exact steps, as that is done at your web host. They need to provide and install an SSL certificate for your website. If they can’t provide one, Cloudflare can (that Origin CA certificate), but your host still needs to help you install it.

thx but can you at least tell me approximatly how for exemple step1, step2…