It would be no different than e.g. the existing payroll tax, or income tax withholding. You'd just notice it as a reduction in your paycheck. And if your income is high enough, the UBI can simply be a tax credit.
We can use the system that we already have, that is, income tax. It has worked fine for quite a while.
Not saying these issues can't be addressed, just pushing back against the notion that UBI is simple to implement.
I think the desired effect is simply a refundable tax credit, and then you can adjust the existing progressive tax rates as needed.
In the model proposed by Milton Friedman, you start with an exemption amount, and a specific proportion of any unused exemption is refundable.
The first dollars you earn reduce your exemption, but at least they aren't taxed, until earnings equals the exemption amount, at which point you are getting no subsidy and your next earned dollar starts being taxed at the lowest rate.
The effective tax rate on those early dollars would be equal to the subsidy rate. For example, an exemption amount of $20k and a subsidy rate of 40% would mean earning nothing would grant you a $8k credit ($20k * 40%). Earn $10k and you would get a $4k credit ((20 - 10) * 40%). Earn $20k and you would get no credit and pay no taxes. But effectively your tax rate on the first $20k earned is 40% -- because you earned $20k and only netted $12k in additional income.
If instead you just give everyone $X, and just have the first dollar earned get taxed at whatever the lowest rate is, you don't have the same problem with regressive rates.