The easy part is the mixture: E85 is nothing but a simple mixture of 15% unleaded gasoline and 85% ethanol.
The hard parts of making gasohol include, operating a moonshine still without getting arrested or killing yourself. This is no joke. The ethanol portion of gasohol is nothing but White Lightning Moonshine.
To make that, you need a distillation column. You grind a grain or cane to make a mash. Ferment the mash. Distill the alcohol from the mash. For fuel purposes, the distillate must be condensed and fully vaporized, in order for the product to be completely dehydrated.
Distillation of grain alcohol is a fine art, whether for libations or fuels. It is also hazardous, be careful!