This article is a draft.
Abstract: jwz is right. Do it the jwz way. However, it is harder to do this if you are using btrfs or lvm. Probably just don’t use them.
Firstly, I am talking about backing up your laptop, which probably has one disk in it. You want to keep going when that disk fails.
The jwz way is to use rsync to another disk of the same physical size, in an external USB case. Then you can swap it in. You might want other backup systems that keep previous versions, or do it over the internet to somewhere else, but this is I think the most important thing to start with, especially if you don’t have a good internet connection, or any at all, as may be when sailing or travelling.
In the past I found the jwz way simple and easy, but it got more complicated when I tried to do it with btrfs and now lvm.