As for the implementation, I am partial to the proposals of MMT economists like Bill Mitchell. They boil down to an arrangement where certain institutions - mostly local and regional government and non-profits, but the details vary - can post jobs. Those jobs will administrated by the institution itself with minimal central oversight. However, funding comes from the central (monetarily sovereign) government at an hourly wage that is fixed by law.
So it is simply an offer that you can take or not. If there is nothing that you like, don't take anything. The theory is that there are so many useful things that could be done once money stops being the limiting factor, it's extremely unlikely this would happen.
Basic income advocates like to say that it allows you to do whatever work you want to do. Well, the Job Guarantee intends to allow essentially the same freedom as long as what you want to do provides some public benefit.
That said, there are other proposals for how to generate infinite demand for labor at a fixed price, e.g. this one: http://www.morganwarstler.com/post/44789487956/guaranteed-in...