So I'm going to start by talking about the numbers of a guaranteed basic income (GBI), mostly because they work out to appeal to self-interest.
First, my estimate of the cost of the GBI is around $3T/year, and I get there by taking 240M adults and multiplying by the individual $12k/year amount needing to exceed the poverty line. Other estimates I have seen are in the 2 ~ 2.5T range, based on estimating at a household level, but I'll use mine because it's more pessimistic.
Next is how to pay for 3T/year in expenditures. First, some sacred cows need to burn:
Social Security - the idea was to give a safety net for the old and disabled, GBI is THE safety net, this money goes to that entirely. 1T/year [1]
Welfare/Housing/Food Stamps - Like above, the GBI is the safety net, these are unneeded, estimated federal, state, local .5T/year [1]
Unemployment benefits - I haven't found good data on this except a quick note from the CBO giving an estimate of around 100B in fed spending. States bear a lot of this cost, so I'm going to estimate this at .25T/year
Medicare - the ACA requires everyone to get private health insurance, and the government should get out. This should be replaced by a program to subsidize premiums for those with demonstrable need. Currently this is at 1T/year[1], I think the replacement would save .75T/year
So from spending cuts alone we get 2/3 of the funding (and if we go with more optimistic estimates, we've paid for it). Leaving only 1T/year to come up with. So far we haven't touched taxes, so everyone is still earning the same income they already have plus receiving their GBI check for an extra 12000/year. If we raise taxes to put some of that back in, say such that the top 60% of earners put back an average of 8000/year, we get .96T/year in increased taxes. The rest I suggest come from a serious overhaul of capital gains taxes and especially inheritance taxes to close loopholes and reduce the level of trust-fund legacy at the top, and it's a small enough amount to not require particularly large changes.
So as it stands, every adult in the US would get $12k/year, even with an increase in taxes those currently earning good incomes would receive an average of $4K/year extra.
[1] usgovernmentspending.com