Step 4: attatchment
so you should have a front bike without a rear wheel, a rear bike without a steering set of handlebars and three rods bolted through its steerer tube. splendid.
take the rear bike and set it up with that front wheel with the rear axle, and using whatever combination of nuts you can get a hold of, make sure the fork tightens so that the dropouts of it are closest to the axle.
try to get the rear dropouts of the front bike on over the axle. now of course, it depends on what fork you're using, what your rear dropouts are like, etc, but if for some reason your fork doesn't fit with whatever configuration you're using, stretch it.
yes, if it's a steel fork, it should stretch pretty easy, as mine did; just put your foot on one leg and slowly bend it. make sure it fits over the rear dropouts, as opposed to inside of them now, and bolt that middle wheel on as tight as you possibly can!