This is the first tiny house we have built, so it was enough of a challenge to build that the documenting side was let slide to a fair extent. Regardless, I will walk you thorough what we did, and how we did it to the best of my ability.
We used as many natural, unprocessed, and re-used materials as possible, which made it less expensive but much more time consuming than buying everything from the lumber yard.

Step 1: Preparing the Frame

As we used an old trailer frame to build this on, we started by demolishing the trailer, separating out the aluminum to be recycled. The plywood floor was saved to be re-used as the sub-floor. Then we went at it with an angle grinder and drill with wire wheel and wire brushes for the hard to reach parts. As we removed the failing old paint and surface rust we looked for signs of penetrating rust which would need welding to maintain the integrity of the frame. Thankfully, we found none.

So we moved right on to priming with a rusty metal primer, and then a coat or two of quality metal paint.

Step 2: The Floor

As we live near an old steam powered sawmill, we decided to use rough milled redwood 1x6's for the floor, so we ran them through the planner and then the edger to get a beautiful floor. As we were using true one inch thick boards we decided to screw them in with appropriate length screws from the underside so that no fasteners would be visible. To protect it for the rest of the project we covered it with some 1/8 inch plywood pieces we had laying around.

Step 3: Framing the Walls

We decided to frame the walls with rough cut 2x2s, which made it a little more challenging than it would have been to use consistently sized lumber from the lumber yard. Se la vie!
We did the framing with 3 inch star drive screws, which are stronger and much easier to drive than philips screws of the same length. I Highly recommend them to anyone who has ever stripped out a screw, or a few. ;-)

As it is only 6x10 feet inside, we decided to keep the back wall without windows, and so we put a window in the other three walls.
Due to the roughness of the lumber we framed with, varying in thickness by up to a quarter inch, the structure started taking on its own dimensions, which we allowed, knowing that it would give it character.

Step 4: Framing the Roof

As this structure is designed for use around the San Francisco Bay area where snow load is nonexistent, we used 1x3 rafters for the roof and 1x2 framing for the trusses.
As we framed it out we did our best to remove as much weight as possible while creating a sturdy movable structure, thus the use of 1x2's, 2x2's, and 2x3's at different places in the frame.

Step 5: Siding With 1/8 Inch Plywood

We had a big stack of 1/8 inch ply ready for use so we decided to skin the structure with it for shear strength (to prevent twisting and increase rigidity). Unfortunately the sheets were 5x5 foot, so we had to cut them to fit our random stud spacing and use two to reach the top of the walls. But they were free so we didn't complain much. They were fastened with narrow gauge staples fired from a compressed air staple gun. This was very quick.
We chose to overhang the sheets in the corners and trim them later with a sharp handsaw. We did the same with the windows and roof line, but using a Sawzall where a handsaw wouldn't fit.
The roof was decked with 1/2 inch plywood for strength and lightness.

Step 6: Lap Siding

For the exterior siding we used rough milled cedar and fir boards of an average thickness of 1/4 inch. They ranged in width from 5 inches to 10 inches and we mixed them up randomly to give it an organic feel. These were stapled on the same as the plywood, but lapped so that the staples were hidden.

Step 7: Insulation and Interior Siding

In the end we decided to go with recycled blue jeans insulation as it is much more likely to stay evenly distributed in the wall than loose wool. Although it is twice the price of fiberglass, it is so much more pleasant to work with that it was well worth it. It doesn't cut with a knife like fiberglass, but can be pulled apart with your hands. As the walls are going to be breathable, we did not want fiberglass particles getting into the air inside the room.

For interior siding we chose the boards with the fewest knots and ran them through the planer and edger until they were straight and beautiful. Then we tacked them up with a finish nailer.

Step 8: Windows and Door

As we were re-using the old trailer's aluminum windows we made a frame with rounded bottom corners to fit it out of old-growth redwood. It was tricky and it would be much simpler to install normal windows from a house.
The door was a very tricky thing. We decided that it had to be rounded as there were so many straight lines already. This was eventually accomplished by cutting the curve out of a big slab of 2x12 redwood, adding other framing and then sheathing the outside with 1/8 inch ply and then covering both sides with planed cedar, fir and redwood thin boards.

Step 9: Roofing

I was not present for the installation of the roof, so there are no pictures, but I can tell you what was done. 8 foot sheets of 1/2 inch plywood were laid across 1x3 rafters, over the already in-place ceiling and insulation. We used a fairly large overhang, about a foot all round to make it look less like a box the way a lot of tiny houses do. This also protects the walls, door and windows from rain. We bought metal roofing to size for about $240, which should last a few decades.

Step 10: Trimming It All Out

We wanted to use at least a little raw wood which we cut down ourselves, and so we quartered a 3 inch fir tree on a band saw, then rad it through the edger many times until it was a good quarter round. These were put in the corners of the room. The window boxes were trimmed with 1/4 inch fir and most everything else was trimmed using old growth redwood. The facia boards are cut wavy for cuteness.

Step 11: Finished!

Well there you go. Probably $1500 in raw materials and hundreds of hours processing them and putting it all together, not to mention figuring out the inevitable complications that come up. We are now advertizing it on SF Craigslist for $8500 and are hoping to make more to-order in the near future. If you are interested, check us out at:
organicbuilding.webs.com

I hope this inspires you to build your own tiny house, don't forget to have fun....