Some car owners in America or Brazil do nothing at all, they just buy it and put it in, but that's not really a good idea although ethanol won't actually hurt your engine the ECU might strain the fuel pump and cause it to blow. Often the sensors will adjust the timing and the volume of the squirt, for it to run, but the extra fuel and advancement of the timing will be totally missing so some cars will drive well and some are terribly flat. The very least you should do is if it has an ECU re-map the ECU for ethanol, after all the major manufactures spend millions and you could use that research just by adding the fuel data. Without doing that you will use more fuel than you need and it won't accelerate quite as well as it could and the check engine light will come on. Obviously the lower the engines compression is the worse the MPG will be. Some Far Eastern cars have high enough compression so the problems will be less, but they'll still be there. Without the remap it might even perform better than it did on petrol, so you'll never know how good it could be.
In a modern car one with an ECU the thing to do is replace the currant Petrol/Gasoline ECU map with a map that allows you to use ethanol. Most car builders copyright the firmware they fit and the modification software So the ignition needs to be retarded two degrees and the mixture changed to take account of it's different energy density, roughly 1 and a 1/3 although each engine design will be different. Some places ethanol has absolutely no illegal exhaust products in the UK you could fit a map that makes the engine turn out the best power or the best economy it can, if you don't live in the UK some of the exhaust products might be tested for, but the amount that's made will be minute.
If a car was built before the late 80's or very early 90's and equipped with a distributor and a carburettor or some carburettors as cars then often where, it will be less technical, not involving electronics. Conversion of a Ford built car would likely be quick and easy simply a matter of twisting the distributor by two degree's to advance the timing (retard the ignition) and re-jetting the carburettors to make the fuel mixture 25-40% stronger, as coincidentally the seals and rubbers used by the Ford corporation tend to be ethanol resistant. Vauxhall's less so because it was originally a British company and only bought by GM in 1925. British cars were never alcohol resistant as Briton didn't have any alcohol for fuel tradition. That is just a partial job making in effect a Petrol engine is made to run on ethanol. Optimising which involves increasing the static compression is truly converting an engine to ethanol that step is rarely done, but it makes the engine unable to use petrol again as the compression is much higher than petrol can accommodate. High octane seems to be a function of the size of the fuel molecule as butane and propane have it too.