Which I do, of course, four times per year.
It's a fairly complicated process. Every time I make a sale to any of the 28 countries, I have to work out how much of that was for the service, and how much was VAT, because I charge a common price across all countries. But the VAT rates vary from 15% up to 27%, so I have to work backwards to the amounts.
Then I add it up for each country, and fill in a form "VAT Moss" to tell HMRC how much I sold in each of the 27 countries (EU but not UK). The UK is done on a separate form.
Until recently, the periods for UK and non-UK weren't synchronised, which caused no end of additional complications, but I worked out how to make them both cover the same periods, which helped.
Of course, most of these calculations are automated, either in my billing system, or in spreadsheets that I've set up. So it's not too bad - I'm not complaining. Well, I am, but death and taxes are inevitable.
But on Brexit, you can be sure that all this will change, and I'll have to rewrite my billing software, spreadsheets and other stuff. As will the millions of other small (and large) businesses in the UK.
I'm not looking forward to that.