I used about a 26 inch bike wheel, but think a kid's bike wheel might work better, being less bulky. I've used them without the tire or tube too, and they work okay, but the inflated tire seems a little nicer...
Remove the handlebars and front fork from the rest of the bicycle. (The handle bars, which are the old fashioned 3 speed kind in this photo, should come off when the strap holding it on is loosened and pried open a bit. The fork is a tougher customer, but should come with exertion and tools...
The next step is to bolt on the main part which extends
from the frame to the handlebars. This is so you don't have to bend
over to use the tool. I used a shovel handle, but sturdy wood of
any sort (even 2 X 4) should work. Bolts and nuts through drilled
holes are best to make a tight non cracking connnection. Here's a closeup
of the connections at the wheel.
There are two cap nuts on the top, showing where the frame is secured in two places to the handle. The braces to the hoe are secured with one bolt, as the correct angle needs to be adjusted somewhat through trial and error.
The handlebars are bolted on near the end of the handle piece (which is a little over 2 feet long). The two braces which run down to the hoe are attached there with a hose clamp. These braces' length is determined by trial and error--long enough to force the handles up to the person's midsection heighth.
There's only room for one good bolt through the handlebars, which means that when you apply force to the handles, it could turn. To counter this, a piece of plywood (3/4 inch worked well is attached up close to the handle bar, to add support to it. Sheetrock screws were sufficient for this...
The hoe was a piece of strap steel, double bolted, and sharpened with a file. I heated the bar with a propane torch before bending it into a squared off U shape.
When using the tool, if the weeds are small you can just
push it along. If they're larger, a back and forth chugging sort
of motion helps work through it. Being handbuilt expect a few
areas to work loose, require reallignment, etc.