Steering Methods

"I am really interested in building my own tall bike. ... I’d like to educate myself more on steering systems. I’ve seen a few different styles, some more appealing than others. The first one is where the handle bars on the lower bike are left on and the fork from the upper bike is welded straight to the handle bars. ... The next one I’ve seen is where you take the handle bars off the lower bike, chop off most of the fork from the upper bike, and weld a pipe from the base of the fork down to the kneckset. ... I was wondering if there was any other ways to go about it?"

 
There are many different ways to steer a tallbike!
 
I've never been a fan of welding the fork of the top bike to the bars/stem of the lower bike.  It's fast, but it looks like hell.  You may LOVE the look though, and if so, DO IT!  Arthur's Bike is built like this.  (P.S. Arthur is running for Governor of Wisconsin!  Vote for him in the May 8th primary!)
 
Chopping the fork arms off and welding a tube from the top frame's crown to the lower's stem is certainly a cleaner look, but comes with the same serviceability issues.  To replace the bearings, you have to chop these apart and re-weld.

We have only really used one of two other options:
 
By far the quickest setup that provides the most serviceability in the future is the 'kinky' steering I used on my first tallbike.
It's not very pretty, but it's straightforward, easy to do, accommodates very slight misalignments, and you can take the system apart to service the bearings.  The head tubes remain completely unmodified, with both bearing sets intact.
 
Pretty much every other tallbike we've built has used a 'lengthened head' method.  You might also call it the 'pitchfork' method, because the fork becomes so long it looks like a garden implement when you're done. :D
It's the cleanest method by far, but takes the longest time of any other method.
Barry's bike, Pete's bike and Erik's bike are all build this way.
 
Two parts must be lengthened - the fork steerer and the head tube itself.
First, the two frames need to be lined up accurately and their head tubes connected.  You can use some bicycle tubing if you can find something similarly sized to the head tube (it doesn't have to be perfect), but I like to use 1 3/8" chain link fence top rail - you can get it at Home Depot or Menards.  If you've got a Huffy bike, then the top rail can be hammered right inside the head tubes - almost no head tube alignment is necessary!
 
Align the frames by clamping 2x4s on either side and maybe a broom handle inserted into the head tubes.  A straight edge or long level is useful to ensure the head tubes are aligned accurately.  Slide the top frame down until part of it touches the bottom frame (usually the bottom bracket/crank will touch the seat post of the lower bike) - that's where you'll weld them together in the back.
When the frames are aligned and tack welded together, you can cut the new head tube piece to length and weld that in.
 
Now, you figure out just how much you have lengthened the head tube (measure the difference between the original head tube you took the fork from and the new, huge head tube length.)  That's how much you'll need to lengthen the steerer.
Cut a piece of 1/2" plumbing pipe to lengthen the steerer.  You may need to taper the ends of the pipe a bit to get them to fit inside the steerer.  (Note that enough of the inside must be kept clear at the top so you can insert the stem to the minimum insertion point)
 
Using the straight edge again, carefully align the new steerer, making a few small tack welds and checking alignment repeatedly.  The threaded part at the top of the pipe should align with the lower, unthreaded part of the steerer at the fork end.  If the threads are misaligned, then your bearings will wear out faster.
 
Once everything is aligned and welded together, grind the large globs of weld off the steerer so that it fits through the head tube without rubbing and you should be able to (hopefully) re-assemble the new fork into the enormous headtube.  If the head tubes are different sizes, you use the top bearing from the top bike, and the lower (fork) bearing from the lower bike.

Depending on your frame geometry, you may end up with a 'wheelie machine'.  We had to modify Barry's bike to increase the wheelbase and keep it from going ass-over-teakettle every time he stood on the pedals.  Normal tallbike antics.