You start by identifying the reasons behind why your application/service may fail and then design and implement the infrastructure for it, that can withstand certain failures for a cost you can bear. If a failure of a piece of infrastructure costs you £1 per day, you might be OK with paying £5/day for the infrastructure to handle such failure. But would be be OK with paying £50 for the same thing?

It's all the matter of defining requirements, then solutions and tradeoffs of those solutions and then implementing it with best practices in mind (automation, testing, monitoring, backups, etc.).

Hit me up if you want to discuss it over a pint! :)