> You are probably talking about the government borrowing it instead of individuals. This tends to not end well.
It tends to end with inflation eroding the debt. Which is exactly what we want in this case, ideally over time rather than all at once, but still.
> The money has to come from somewhere.
Why can't it come from nowhere? That's where the debt it's replacing came from. Banks don't actually have the money they loan you. They're only required to keep a small fraction of it in reserve. The rest of the money is created from nothing.
We're fundamentally in a position that we have to print a lot of money and cause a nontrivial amount of inflation because we already did that throughout the housing crisis and the response to it, allowing the prices of housing, education, healthcare and stocks to enter serious bubble territory. As a result of that either the nominal prices of those things have to come down or nominal wages and the nominal prices of everything else have to go up to reach parity with them.
But lowering the nominal prices of those things can't save the people who already paid for them on credit and now hold significant debt as a result, which is nearly everybody. Moreover, reducing nominal major asset prices tends to cause much more economic damage than raising the nominal prices of some other assets, because it causes people to default on underwater mortgages, hoard cash rather than investing it because the currency then appreciates in value, etc.
So inflation is good right now and printing the money would be a completely reasonable thing to do in the short term.
What happens after that is you pay for it from taxes, the same as the subsidy for [more specific program] would otherwise. But that should be a lot easier afterwards because real wages (and therefore real tax revenues) should be significantly higher than they currently are relative to the cost of necessities like housing, education and healthcare. In other words, the real amount of UBI you would need would decline with the real cost of those necessities.
> Wealth transfer schemes are inevitably unpopular and tend to not work well.
Then why do we keep passing/expanding rather than repealing various redistributive welfare and assistance programs?