Let’s assume I buy a .omg.lol domain.
How can I then map an existing non-omg.lol domain from a registrar onto it, such that I have the fastmail bit for mail, and the omg.lol bit for page hosting and weblog and all the other goodness?
At the moment, the help section on DNS does not seem to be very instructive in this area.
Do the following:
1. Go to your registrar, and wherever you can add a DNS record, add a root A record for omg.lol’s load balancer IP(5.78.24.5). Save this, then check that the A record is properly propagating my putting in your domain name at What’s My DNS?.
2. Next, go to your Switchboard, and add the domain at the bottom of the page under “Add an External Domain”. Then click Save.
3. Wait for omg.lol to provision a TLS Certificate, and you can also set where it points to(you’ll want yours to be set to “Web/Profile Page”)
4. Then, follow the instructions here to set up your Fastmail. I don’t use FM, so I don’t have any extra tips here, but that should be it! 
Thanks - and I’ve just realised now that you mentioned the Switchboard page, that there is help for this, but it’s in the section for the Switchboard, rather than under DNS…
Waiting for things to happen on the propagation, but thank you for setting me in the right direction!
Happy to help! And yeah, it’s a bit annoying that it’s under it’s own specific help page, pinging @adam , maybe these should be cross-linked to each other so that it’s a bit clearer how these are linked?