DKIM/SPF for site on shared hosting, gmail

I haven't done this before and I haven't quite got my head around what is actually happening.

I've got a site on shared hosting, minimal traffic.
Plesk control panel, domains managed by the registrar.
SSL set up for the site.
The site sends emails when people fill out the contact form.
Humans use Gmail to send and receive emails.

Plesk lets me use DKIM spam protection
View attachment 243956

I've seen info about using puttygen to generate the keys I need & it seems pretty ad hoc….

DKIM/SPF for site on shared hosting, gmail