UBR is interesting as a kind of though experiment I guess, I don't see it being relevant any time soon.
Even the people who support it usually do so in a a vague "eventually we'll need it sense" and not a specific proposal to make it happen now.
Step 1: Start at $100/month. $1200/year. Drop some cash welfare benefit by $1200/year. Lower the amount for the 25% tax rate, digging into the 15% rate, by $12k.
Result: You make more than $37,951 you're exactly equally off. If you're receiving some cash welfare benefit in excess of $1200, you're equally well off. If you were making below $38k but NOT getting the cash benefit, you're slightly better off.
Continuing: Increase by $100/month for ~5 years, ending at a payment of $500/month, though I wouldn't cry at $600. Eventually you get rid of non-healthcare related welfare payments, including non-cash versions. If you expect for people to pay for their own healthcare, payments would have to be higher, and probably sequestrated into some sort of 'healthcare only' account to make sure it isn't diverted. Continue dropping the lower 'introduction' tax rates, eliminate the personal exemption, and welfare programs, starting with the most 'cash-like'.
End result: The lowest tax rate is 25% and the personal exemptions are gone, but that's okay because you get the equivalent of a $6k refundable credit. If you earn somewhere in the $100k range your finances are within a couple bucks of what they were before. If you were on welfare you aren't anymore, though you get $500-600/month to do with as you please. More importantly, you're not going to lose that money if you get a job, so there's less incentive to sit on your butt. We also need to hire a lot fewer welfare agents to watch you sit on your butt to make sure you don't make something of yourself while still receiving the benefit. Bonuses all around.
Less government, cheaper, check.
Less control over people's lives, check.
Government getting stuff done at minimal intervention, check.
Drastically less bureaucracy needed, check.
Flat(ter) tax rates! check.
That's how a libertarian does a BIG.
Now, to go look at the rest of the comments...
I'm not dismissive of historic trends. I just don't have enough blind faith to believe that historic trends never change. Akin to "housing pricing always goes up".
Good example. Horses. Historically, constant technology improvements made their lives better.
Actually, not all live in comfort now, with the closing of horse slaughterhouses, it was discovered that the slaughterhouses put a minimum value on horses - so the ones that didn't go to the glue factory were well taken care of because they were above that base value. With that option gone, the value of a horse can easily go negative - it's an expense to get rid of a horse anymore, and thus you see far more neglect. In some cases you can't give them away, and they're still big animals that need to eat.
Ironically, closing the slaughterhouses to be 'more humane' led to far more cruelty against horses.
Technology has sharply reduced the number of lawyers, manufacturing, etc. So far, economics show that it has not led to an increase new better jobs for people. The economists were absolutely shocked at this. They counted on conventional optimism as well, and the numbers haven't backed it up.
Better production per individual can lead to better pay for all, but in the latest boom it's been the owners who have gotten the vast majority of the gains, thus why the incomes of the rich have been going up while the labor of the poor has declined in value.
I'm not even going to argue the merits or demerits of having a UBI system. The mere fact that certain people are confidently predicting the need for it is de-facto assurance that they will be wrong.
I'd like to see it, done properly, as I think it'd be drastically cheaper and more effective. By eliminating welfare cliffs, we can shove these types back into the labor system far more easily.
Oh, as for UBI. It won't work if one does simple math.
So you mean I need to break out the calculus?
$3,180,000,000,000 360,000,000 = $8,833.33 check for every person. If you eliminated every single other cent being spent by the US government.
I posit about $6k/year, which if you get 4 adults to team up is $24k, or the poverty line for the US.
Good luck living on that. US poverty line is $12,060. In my area, minimum living wage is allegedly $21,165.
Then they need to move out and make room for those that can earn the $21k needed. Or your area can pay a differential if they want the poor people still around to exploit with low wages.
$21k, as is, is way too close to the national median income.
For poverty 'basic income' you would need 4,341,600,000,000, a measly 1.4x increase in taxes.
For minimum 'basic income' you would need 7,619,400,000,000, or tax revenue would have to be increased by 2.4x.
Here's the thing: Electronic money transfers are incredibly cheap. As I mentioned in the plan above, what you do is get rid of the lower tax brackets, such that for the vast majority of people, the amount they pay in extra taxes under the system is negated by the BIG they receive.
If you want to continue normal government stuff, you're looking at 2.4x increase for poverty basic income. 3.4x increase for minimum basic income.
Why? For one, $12k-$20k/year is ridiculously generous. Two, you're ignoring all the federal welfare programs that could be eliminated. Three, you're not figuring on increasing taxes on the lower end, to tax back the BIG amounts gradually, resulting in no welfare cliffs.
I tend to pay 30% in taxes. I'd personally face a 72% tax rate. Or 102% tax rate. In return for minimum living standards.
What makes you assume this? How do you figure this? Is the 30% your marginal tax rate, or overall? You're not including adding the BIG amount back into your income?
The notion is that you'd be able to discontinue paying for social services. By just handing out money. Which assumes people not capable of financially, mentally or physically supporting themselves would instantly BE able to financially, mentally or physically support themselves. Which is obviously not statistically possible.
Because it's a monthly amount, the odds are that they'll figure it out sooner or later, usually sooner. Because it's monthly, unlike lottery winners, they can't spend all of it immediately. Generally, they'll figure out ways to optimize their income, just like they figure out ways to take encumbered money like food stamps and turn it into unencumbered money they can use to spend on anything.
All of the guaranteed income people are Democrats. (please correct me if I am wrong) The DNC can't even pay minimum wage to some of its employees.
Why should I believe they can provide a universal basic income to our citizens? Am I missing something? 
There are actually libertarian guaranteed income people, though we approach it differently than the democrats do. They approach it as a human rights thing. I approach it as being ultimately a money saver as well as personal freedom increaser.
