I cut and bent the sign in such a way as to try and capitalize on the inherent properties of the material, namely its tensile strength. Since I designed it as I went along, the substructure is a little weird. I just kept adding pieces until it stiffened up. With a little more forethought, you could organize the structure better, but I kind of like how it developed organically, and the final form reflects that.
That said, it is still surprisingly flexible, and conforms to your body some when you sit in it. At about 13" off the ground, it is a low, lounge-type chair. Given that the wood is already weathered, and the sign was meant to be outside, this chair is great for a porch, patio, balcony, or deck. Originally 2' x4', the dimensions aren't ideal for a chair, in that the seat pan is a little long tailbone-inside-of-knee wise. However, when you sit in it, the big 45-degree cutbacks on the front edge are where your legs go, and it works out fine that way. It is really comfortable and incredibly lightweight -- you can pick it up with one hand.
I don't expect you to find the same sign, or the same dimensioned sign, so I'm not going to include any dimensions. In true readymade style, I measured everything just by holding it up and eyeballing. You can do the same . . . .
Step 1: Bending the Metal
I marked the sign into two 2' square halves. Off that center line, I made two sets of two 45-45-90 triangles with legs of 5", 5", and 7". Snip down that center line 5". Then carefully line up the hypotenuse with the edge of the workbench and hammer it down with the mallet. To get the cleanest bend, hit right on the edge, not down at the bottom of the piece. Repeat for all four triangles.
I did the same at the corners, but those triangles have 8" legs. For what will eventually be the back, hammer them down all the way flat. Leave the triangles at the front of the seat at 90 degrees or so.
Last, bend the whole thing in half along the center line by pulling the centerline triangles together until they completely overlap. Drill a hole through both and bolt through to hold the sign that way (see the last photo of the intro to see those bolts up close).
Step 2: Legs!
Each was made from a signpost that had been ripped from a 2" x 4", down to roughly 1-3/4" square in cross-section. To get the miters right so the feet hit the floor, i propped the bent sign up on some boxes and just eyeballed it by holding up the wood. Drill a hole in the foot of each piece, a couple inches up from the bottom, for some tension cross-bracing later.
To help attach the legs, I ran two rails from front-to-back. First, tape them underneath the sign, then flip it over, drill pilot holes and screw through with drywall screws. It is important to drill pilot holes because the wood is thin, old, and dry, and splits could be fatal.
Put another screw, say a 2-3" deck screw, through the sign into the end grain. Bend down the triangular flaps and screw them into the front legs as well. Don't worry if the legs are still kind of floppy at this point, we'll take care of that.
Step 3: Bracing
The tension brace that runs in an X at the floor end of the legs is made with nylon string. Tie wire or rope works just as well. This brace does a couple things: it prevents the tendency of the legs to spread outwards from the center when weighted, it tightens the legs against the top joints where they join the sign, and it can exert a lot of force on the still-flexible chair, straightening it out so all the feet hit the floor evenly.
The brace is made similarly to a tourniquet: run a loop of string from corner to corner, then put a dowel or bolt or something in between the strings and twist until taut. Tie it off at the intersection to keep them from spinning out.
Step 4: Back
Then, four verticals run from the back legs up to that rail. They are staggered, so a horizontal could be inserted and pushed up until tightly wedged into place, then screwed through. You can see in the fourth picture it runs in front of two of the verticals and behind the other two, which really tightened things up. A center rail comes down and attaches to a last horizontal running crossways between the back legs.
Finally, I trimmed down bolts and too-long screws with a Dremel. I also ground away rough spots and edges, because that metal can be killer. Clean it with denatured alcohol (careful, depending on the age of the sign, the alcohol can strip some of the paint) or 409/similar general purpose cleaner.