Minimize Usage Realistically?
Anything that consumes a lot of data would be best left to the bonus zone. Things like software updates, for instance, or downloading large files, like a movie.
I would suggest installing the free Glasswire software to see what's using the data. The software only measures data usage from each device it's installed on, and it's available for Windows and Android. There are routers you can get that measure usage for the whole network, so one of those may be useful, though they are expensive.
If you have 10GB per month for the household, this means that the household can use no more than about 1/3 of a GB per day in a 30-day month, a bit less in a 31-day month. That's a very small amount and would likely hold if you only limited yourselves to browsing and email. If you stream anything, that would eat your data very quickly, and you'd have to seriously budget for it by not using the data for much else. A movie in HD can consume 3GB per hour. You'd be out of data in less than 4 hours.
You'd also need to lock everything down to prevent automatic updates, and if you're using anything like a Roku or a smart TV, for example, you'd have to disconnect it from the wifi, as they use data even when you're not actively using them.
Though some of the information in the following two posts is a little dated, they can still be quite helpful...
For your computer(s), I would install the Glasswire that maratsade referenced, and for the browser on the computer I would install an ad blocker. Ads can use quite a bit of data, especially when they're the type that continually show new content rather than still ads or video ads that play only one thing. One of the most common is Adblock Plus, and it works very well. It even stops those annoying ads from playing in Youtube videos. You just want to make sure to go into the options and uncheck "Allow Acceptable Ads".
With computers and other devices, it's important to lock down automatic updates, both for the installed software/apps and the device software itself, such as Windows. It's best if YOU control your updates, data wise.
It can also help to set the network in Windows computers to a "metered connection". This helps to ensure that data is not wasted by the system itself. Other devices may have a similar setting for their operating systems.
I can tell you that, with one main Windows 10 desktop computer and one Windows 10 laptop that I don't use very often, but I do keep up to date, and not much media, such as Youtube, I have to be careful to not exceed my 20GB. I've no doubt that with half of that I'd go over every month. Devices are using more and more data on their own these days, and everything we do on the net is going in the same direction. That 10GB used to be enough, but isn't so much the case today, isn't all that surprising.
