Unfortunately, we haven't even figured out how we're paying for our existing budget.
The nice thing about an adult UBI is that it doesn't distort the market as much as our other social programs, and if its introduction means the elimination of medicare, medicaid, social security, TANF, student loans, unemployment benefits, etc. (see https://en.wikipedia.org/wiki/Social_programs_in_the_United_...), then that's a win in and of itself.
Some of the distortions I'm referring to: TANF incentivizes having more children to get a higher payout, unemployment benefits incentivize staying unemployed longer, student loans increase demand for expensive tuition (and hence each increase in loans/aid results in a relative 60% increase in tuition), etc.
To work, UBI has to be truly basic (i.e. most people would not be satisfied living on it and seek some form of supplemental income), which is difficult because people will vote for the Bernie Sanders' of the world who want to raise it -- especially retired folks.
I would prefer a negative income tax that has inherent balance. I don't know of any good white papers, but I've given it a fair amount of thought. Consider a negative 'flat' income tax such that the rate above a certain point is X% (say 15% tax rate on income above 40K) and (100-2X)% refund (e.g. 100-2*15=70% refund on income below 40K, so if you make 30K you get a 7K refund). If the formula is constitutionally locked in, and only the rate is subject to change, then voters have less incentive to raise taxes because it decreases refunds. The only incentive I haven't figured out is how to avoid getting people to vote to raise the cutoff point for paying taxes/getting a refund. Perhaps it should be locked at the (100-4X)% percentile, so with the above numbers the poorest 40% of the population would not pay taxes, which would probably put the cutoff closer to 30K. All of these numbers are arbitrary, and for the examples I've given it would basically lock in rates of 10-20%.